Even Intervention Pact sees occasional play and saw a spike when Madcap Experiment decks began to be brewed. But the red Pact, Pact of the Titan, hasn't seen much play since being a part of some Hive Mind Legacy lists back in 2011 to 2013.
Hive Mind decks aren't really much of a part of the Legacy metagame anymore, so Pact of the Titan went by the way side. It's easily the weakest of the Pacts. While creating a 4/4 creature at instant speed for no mana sounds awesome, having to pay 4R at the next upkeep is rarely worth it. However, when Pact of Negation, Slaughter Pact and Summoner's Pact were reprinted in the original Modern Masters sets, Intervention Pact and Pact of the Titan were not. This meant that their supplies have remained relatively low.
So it was only a matter of time before Intervention Pact's price spiked. But while that spike make some sense, when Pact of the Titan was bought out on TCGplayer on 10/29/16, many heads were scratched. The TCG Player mid price hit almost $7 before falling to about $6 a couple days later. But no one is really biting at this price, which means it's pretty clear that this was an intentional buyout for speculative purposes.
Is this card ever going to be good enough to justify more than its original price of about $1? It's doubtful. This was really just a case of someone buying out a card that had low supply that may see play again at some point. This card is fine for what it does, but it's not something that most decks want. Some Commander decks will play it, but it's not something that you're going to see on a regular basis. This is one I'd sell high on if you happen to have any copies laying around. There should be a race to the bottom sooner or later, but this card will now always be worth more than it was before.
June 2017 Update: Pact of the Titan still has a market price of almost $5 on TCGPlayer. I would still recommend selling this card, as I can't see it maintaining this high of a price long-term. Although, after being at this price point for a long time, it's not anything you have to sell in a hurry.