I suggest reordering "a card costing up to $6 from the supply that was gained this turn" into "a card costing up to 6$ that was gained this turn from the supply"
Two points:
• I think all official cards say where to exile from so this technically would allow you to exile a card from somewhere other than the supply, no? This could probably be explained in the FAQ but is not consistent with the official cards.
• It's minor, but it is also functionally different, in that with the original of someone gains a card from the trash, you could exile a copy (from the supply); with the new wording you can't. Like I said, minor.
I think I'd suggest going with "a card from the supply costing up to $6 that was gained this turn" just reordering the "from the supply" and the "costing up to $6".