This popular land card has already seen its floor at 3 bucks most likely. While it could dip below its current price as rotation occurs, there's so much casual demand for it that it will probably be the most valuable non Thoughtseize rare for a long time.
This set's strength is definitely in its mythics. In fact, all of them are pretty much buys with the sole exceptions of Stormbreath Dragon and Hythonia the Cruel. Previously a standard staple the storm breath seems modern playable but at five mana players would likely prefer Thundermaw Hellkite to it. Taking a look at the tepid buylists, it could drop to as little as 2 dollars before casual dragon lover demand helps it grow again long term. I'd call the dragon a hold.
Hythonia the Cruel sees fringe Commander play but is one of those flavor mythics that just really is true bulk. If you can have her for a quarter that's fine. She's already bulk so she's a hold.
My favorite card in the entire set from a financial perspective is Ashiok, Nightmare Weaver. At a low of 6 bucks right now, Ashiok has seen some rogue modern play. I think the weaver of nightmares has a lot of potential in a wide variety of decks, exiling chunks of your opponent's deck while stealing your opponent's best creatures. With the power level of modern creatures being so much higher, this competitively under-appreciated planeswalker has lots of room to grow.
The Theros gods of the set all have lots of casual applications. Purphoros and Erebos are probably the most popular with Nylea probably a close third. Thassa sees some Modern play and is excellent in commander for both her scry and unlockable abilities. Heliod is easily fifth but he has his fans, especially for enchantress decks in commander. They're all good buys that can be had for considerably less than their TCG mid prices.
Xenagos the Reveler is a pretty cool planeswalker. At 5 bucks he seems like a good buy, but as he's played in red green devotion in standard, it's good to be cautious that he'll drop a couple more dollars due to a mass sell off. But his strong buy list prices suggest he's already close to his floor. He seems to be modern playable but like Domri Rade from Gatecrash they don't currently have decks to slot into. Casual demand should be there for this walker though.
Master of Waves is a modern and legacy Merfolk staple. Why it's only 4 dollars is beyond me. Mono blue devotion isn't a thing in standard anymore so it seems supply is just really high. As I see this being a mythic most players will just hold on rotation I think it's high tide - err, time - to grab a playset.
Polukranos, World Eater is an absolute monster but is he modern playable? At the very least he's under 2 bucks and mono green commander decks love to utilize his monstrosity ability.
Ashen Rider is excellent in legacy reanimator. It has its uses in Commander as well. It buy lists poorly so you can probably get it at bulk. Its effect is super powerful though and modern may one day have room for the rider. It's a long term investment for sure but as near bulk mythics go it's a solid keeper.
Medomai the Ageless is a very strong commander card. Taking extra turns is awesome. I think foils are where to be on this guy though at 2 to 2.50. It's another one to hold long term.
Underworld Cerberus has shown potential in the past but after some fringe play on standard it's been shunned by constructed players for quite some time. At under a dollar though, and playable in a number of commander decks. It's another long term wait and see card that basically can't lose money.
Whip of Erebos and to a lesser extent Bow of Nylea have been constructed playable. Being both enchantments and artifacts actually works to their advantage in commander and despite their low list prices they sell well. It's fine to sit on playset of these. Whip has had a Clash Pack foil printing which may hurt its growth a bit, though. Spear of Heliod will probably hit true bulk at rotation but should be a solid hold after that. An anthem that punishes an opposing creature for dealing damage to you is solid value in Commander.
Hero's Downfall is solid removal in Commander. It's been very good in standard but isn't as widely adopted in modern. It probably won't fall far, though, as hitting planeswalkers is quite good in commander and casual settings.
Colossus of Akros continues to defy logic by buy-listing well due to outstanding casual demand. Being an indestructible artifact that can become a 20/20 is really strong in commander. It gets enough play to eventually be worth a few dollars.
Prophet of Kruphix getting an awesome alternate art reprint really hurt this card from growing. Despite being one of the most powerful cards in commander there is talk that it may be too good and could be banned in the future. It does a lot but I don't find it that degenerate. Solid hold.
Outside of Ashiok having a lot of potential in the format and Thoughtseize being obviously good, there isn't a ton of modern playability in the set. The temple scry lands are too slow for the format outside of perhaps the amulet bloom deck. Anger of the Gods is an excellent sideboard card that can hose a few decks in the format so it's a solid hold.
Sylvan Caryatid is a bit too slow for modern as a two drop since most of the best creatures in the format take that space. Good Ole birds of paradise still does the job. But it's hexproof mana ramp in commander so while it may drop slightly at rotation it's a good hold.
Mistcutter Hydra is a solid sideboard play in legacy for its protection from blue. But it's not quite in demand enough to keep it out of bulk bins. Definitely a hold.
Soldier of the Pantheon is a guy I always expected to break into modern with his 2/1 stats, protection from multicolor, and ability to gain 1 life each time an opponent casts a multi colored spell. He can block Siege Rhino all day. He's pretty much bulk at the moment, so there's no risk here at all. I feel that some aggro deck will take advantage sooner or later.
The Sun's Champion has been the queen of Standard for some time. Her duel deck printing absolutely wrecked her value and a six drop planeswalker is just not really playable in Modern right now.
She's fantastic in Commander but I don't know how quickly her value will come around. She's a hold at the time being. Her set foils though are probably a buy with the small spread between list and buylist.
With how so many of these cards are good in commander, it's little surprise that they're doing well financially as foils. The spread is so small on many of them that there's probably considerable growth potential for them all especially the mythics.
Swan Song is an excellent counterspell in legacy and it sees tons of commander play. Pushing 8 bucks it's a solid hold.
Burnished Hart is a commander staple as artifact creature based mana ramp that can slot into almost every deck. Foils at 3 bucks seems low. Definitely a buy.
Foil Master of Waves at 8 bucks? Count me in!
A foil that's only a dollar right now but has a solid spread is Curse of the Swine. Mono blue removal that exiles and is scale-able is oft played on commander. It may never be big money but it's a good hold.
With a good number of mythics likely to double in coming years this is actually quite a solid set to sit on. The playables have all pretty much hit their floors and the foils haven't quite pulled away in price like you'd expect some of them to. Financially it's been a poor set for standard speculation but for a long term spec portfolio, you really can't go wrong with Theros.