Would there be any problem with this?
Halfback
Action - $4
+$1
+2 buys
If you have another Halfback in play, discard this.
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While this is in play, when you buy a card, +$ equal to half its cost in coins, rounded down
It's a sub $2 effect after being played the first time, it's a terminal, and it doesn't draw, so I'm ok with you being able to draw it and play it twice in the same turn. Are there any ridiculous combos that I haven't thought of? Any infinite loops?
Also there are too many cards that it forms game breaking combos with if it's on gain, so it's on buy.