Emptor
Types: Action
Cost: $5
+1 Buy. Reveal cards from your deck until the total cost in coins of the revealed cards is $5 or more. Put the revealed cards into your hand.
I like this with the cutoff at $6. I think it'd be great except I think there should be something tweaked to prevent people going for a deck of 16 coppers an an emptor.
maybe this:
Emptor
Types: Action
Cost: $5
+1 Buy. Reveal up to 6 cards from your deck until the total cost in coins of the revealed cards is $6 or more. Put the revealed cards into your hand.
or possibly this:
Emptor
Types: Action
Cost: $5
+1 Buy. Reveal cards from your deck until the total cost in coins of the revealed cards is $6 or more. Cards costing $0 are considered to cost $0.5 for this purpose. Put the revealed cards into your hand.
EDIT: Either of these suggestions would also make the combo with highway not quite as overpowered. Personally, I think Highway is kinda broken anyway.
I don't think it should have a restriction. Look at KC, a valid restriction for that card would've been "You may choose an action card in your hand
that is not another KC...". That would've been reasonable, KC would still be pretty good. But as we all know, KC-KC is, although slightly ridiculous, really fun. The same goes for this card, sometimes you'll draw 15 junk cards and a forge and get rid of everything, but that's fine. You don't have to nuke power combos.
And if you have a deck full of copper and an Emptor, it's smiliar to Counting House, which isn't broken either. Granted, you don't have to have the coppers in your discard pile for it to work, but it's also harder to set up, because you have to get rid of almost everything else. Even another Emptor puts a stop to it, since it's terminal, you can't play it. And once you start greening you quickly lose steam.
Highway is a different story, but again, it's fun to have ridiculous combos, as long as they don't appear too often. Apprentice/Market Square is an equally strong 2-card combo, and noone complains about it.