While Mavren Fein is a bit over a dollar retail, he's worth spending a few extra pennies for, although you can find the odd copies below $1. There's also a lot of casual demand for this card, as Vampires are a huge deal at the kitchen table. EDH also wants this guy in a few select decks. Therefore, there are multiple sources of demand. Stock is also steadily dropping on this guy. Now is the time to buy in.
Even if Mono-White Vampires doesn't become a top format contender (which it might) this is still a good card. Good vampires can reach $5 and higher over time. Foils aren't a bad buy at $3, either. The reprint risk of this card is lower due to being a Legendary Creature that's in a tribe that just got a featured Commander deck in 2017. If you're willing to drop $12 on a foil playset, the potential reward is pretty high, with so few foil copies left available.
Also, Mavrein Fein is very good with Anointed Procession, and I don't have to tell you how good that token-doubling card is!
Is Mavrein Fein a card that you feel confidence in gaining value in the near future, too? Are there any cards near or under $1 that you believe we should be looking at on MTG One Dollar at a Time?