yeah sure i'll bite. Here's one that works with captain, workshop, etc as kind of an inverse cost reducer. It's not always gonna be worthwhile (when there's no cards that select by price on the board) but workshopping grand markets sounds pretty chill.
edit: fixed to remove some ambiguity
What exactly does "select" mean here? What does "selecting" a card costing $2 more actually do?
so take a peek at Workshop - "Gain a card costing up to $4" - this is selecting a card from the supply that costs up to $4, then gaining it. This lets you select a card costing up to $6 with it.
Or consider Smugglers. If you have this project, your opponent bought a province last turn, you can smuggle a province.
I have a suggestion for a much simpler way to word this:
"When you play a card, you may first have all cards cost less (but not less than ) until you finishing playing the played card."
Same effect, but less confusing as it uses familiar concepts like cost reduction and "finishing playing" a card. Less wall-of-text-ish, too.
I don't think this would quite work (for the 2nd Captained card, at least). From the wiki for Royal Carriage: "With Duration cards, you call Royal Carriage after resolving the Duration card's on-play effects; you don't wait for the next turn." so you finish playing it before the start of your next turn.
How about (and this may also not work for the 2nd Captained Card - see below):
"During your Action phase, all cards in the Supply cost less (but not less than )."
I'm not sure of "the start of your next turn" occurs at the start of your Action phase or before your Action phase begins? (And obviously, this wouldn't affect any treasure that have you select a card (e.g. Horn of Plenty) or Night cards).
"All cards in the Supply cost $2 less except when you buy them"?
Here's one that's less ambiguous but probably too clunky:
"All cards in the Supply cost $2 less, but you must pay $2 to buy a card."
Edit: I just thought of something that's not exactly the same but would be interesting:
"All cards in the Supply cost $2 less, and when you buy a card, take 2 Debt."