Remember that the server's goal is to maximize cool-stuff/core not minimize cores/player(1).
Highly efficient code bases have some of cool-stuff tricks. Since they consume less cores they have a great cool-stuff/core ratio. A rebate that encouraged more cool-stuff has a similarly high ratio. I don't think credits as a rebate for CPU would improve the over all cool-stuff/core ratio. Players would mostly just do less or cash in on the otherwise unused CPU and create a new credit source (bad).
Selling bucket has the potential for lots of cool-stuff. First order: active bucket market place, selling a unique resource, anchoring credit value, new credit sink. Second order: Players building 300+ cpu empires, new bucket intensive tactics, creative space for players to create cool-stuff that would be restricted by 10k bucket.
So how much cores does the new cool-stuff cost? A bit. Every core spent here is new cost, as opposed to shifting cost from some other activity. This is slightly offset by the economy effect of giving credits to players who need it to do more cool-stuff.
Subjectively, this seems like a high cost, possibly high reward feature. I've heard rumblings from other players that the disparity between newer players and established players with a 300 cpu empire, is a design problem. Selling bucket makes this disparity greater for better or worse.
Overall, I'm not opposed, but I suspect there better (less risky) features to add.
(1) If cores per player becomes a true issue the devs can always scale the value returned by
Game.cpu.getUsed() so that each player consumes less server cores. Also I'm purposely ignoring the game design side of the question of balancing cool-stuff among players and encouraging player subscription.