Here is my submission (Card has been updated)
Notes:
1) You may select any card in play (actions, treasures, night). You may also select duration cards from other players play area.
2) If you select an opponent's inherited estate this will play as your type of inherited estate at the beginning of your next turn. If you didn't buy inheritance, it will have no effect when played.
Rationale for price:
I'm comparing this to throne room/crown, and not BoM because it plays more like them. Ignoring it's ability to react on your opponents' cleanup phases (we'll get back to that later), it doesn't look as good as a crown. They both allow you to play itself as a card you already own without taking up an action.
Pros of copy over TR/crown
1) Can play as a night card (or treasure card in the case of throne room)
2) Can't draw dead
3) If you get crown late in your turn with no good targets, it stinks.
4) Can use multiple copies to play as the same 1 action card you have (e.g. you can use multiple of these to play trusty steed)
5) More useful for while-in-play effects
Cons of copy over TR/ crown
1) Takes effect next turn (which severely limits the number of times you can play it, and it also will miss the reshuffle)
2) dead card the turn you get it
3) Doesn't work as well with cards that trash themselves
I think this has use as a BoM variant in games without good draw. If you don't draw that much, you're much more likely to get to use it's reaction on your opponent's turn (thus getting the benefit quicker and not causing it to miss the reshuffle) which will usually allow you to use it as something worthwhile.
What do you guys think of the price?
Update: After enough people saying it, I've upped the price to $5.