This is very interesting. I think it's kind of a Tactician-ish card, where it capitalizes on the principle that one big turn and one small turn in Dominion are better than two average turns; but this is much more long-term than Tactician. Instead of throwing away one hand to get a double hand next turn, it slightly hurts one turn every shuffle, until you decide to explode into a megaturn.
If there are no other Looters on the board (which will be the case most of the time), then each time Excavation comes up (until the megaturn), it will feel like a dead card in your hand. The reward that you get in exchange for having that "dead" card is that later on, you will get one more vanilla bonus added to your megaturn jackpot. Is that worth it? I doubt it. If it gained the ruin, played it, and returned it all in one shot, it would be terrible (buying it would almost be like buying a Ruin); but your version is clearly at least somewhat stronger than that, since it can save them up and capitalize on the Tactician principle.
Consider some of the things you could have saved up on your Excavation mat. If you played Excavation four times (which is like seeing a dead card four times), you could maybe get +4 cards; or +3 cards, +1 action; or +2 cards, +2 actions; or +$4; or +$3, +1 buy; or +3 cards, +1 buy. Those are probably the "best" effects. (Maybe Survivors would be good with +cards? Not sure.) Most of those are roughly $5-$6 effects. If I buy a Margrave, I get +3 cards, +1 buy, plus a bonus, and I get that every time I play it. If I buy Excavation, I have to play it 5 times just to get +3 cards, +1 buy. The lack of control over what you get probably hurts too.
It's a very cool idea, but I'm pretty sure that it's very, very weak; I don't think it's ever worth getting on boards without other Looters (and even then I'm not sure). It's just that spending a buy (and $2) to get a card that's going to be dead in your deck for most of the game is not worthwhile, even if it gives you a decent bonus later on. If you add +1 card that makes it a cantrip; maybe you could do that and bump the price up? That sounds really powerful, but thinking about it some more, I think it's worth testing it as a cantrip at $4. If it's too strong maybe bump up the price or bump down the vanilla bonus (maybe +1 action, +$1). It's tough to predict exactly how strong that effect is, but I'm pretty convinced that it's not strong enough to be worth buying on a card with just +1 action. As it is now, even if you offered to let me start the game with one in my deck, I don't think I would want it.