Many price spikes occur due to what are known as "price corrections" meaning that the card's price relative to its demand finally normalizes as supply diminishes. However, many other price spikes occur to the dreaded buyout, as in one or a few speculators buy out the internet of a particular card with low supply that may or may not actually have future demand. In the case of Intervention Pact, it doesn't seem that many people were willing to actually pay more than $5 for this card when the price spike occurred. So in this case, we can assume this was a buyout target.
Here are two examples from Tappedout.net of Madcap Experiment combo decks in modern: Aetherflux Experiment and Madpact Experiment. Notably, another card that has seen considerable price gains around Madcap Experiment speculation, Platinum Emperion, is not included in these two examples. However, an 8/8 creature that makes sure your life total never changes is pretty significant, although not in and of itself a win condition. You'd rather hit the Reservoir or the Colossus, I would think.
Whether this Madcap Experiment strategy actually pays off in Modern is going to be a big factor in if Intervention Pact becomes a truly $10+ card. While the brewing has been intense around this Red sorcery, there hasn't been a truly optimized list just yet. Personally, I would stay away from Intervention Pact at anything over $5. It is a low-supply card being from Future Sight, the Tarmogoyf (and Horizon Canopy) set.
Intervention Pact is definitely a wait-and-see card. Considering that Angel's Grace is an $8 card, it's fair to say that the Pact could be north of $10 if it's truly a four-of in a Madcap Experiment deck. But it's hard to say how competitive the archetype will be. It's definitely a fun combo deck to try out, and if you add in Blue mana to play disruption such as Remand and card selection spells such as Serum Visions, there is a shot that it can do some work in Modern. There is real potential here, but don't throw your money into the ring just yet.