I think Donald is coming from a different perspective to this discussion than everybody else. He doesn't want to design strictly better cards, and can change the cost of cards to be tested. So of course he cares about costs.
Pretty much everybody else here spends much more time playing Dominion than designing Dominion (but maybe not as much as the time they spend discussing Dominion). For us, coins are commodity, but we don't set prices. So, I have 5 coins, is there any reason I should consider a Village over the Bazaar? We're not here wondering if the price is right, we just wonder what to buy.
Example: I want to buy a TV. There's one for 100€ and one for 120€.
I might ask the seller: so what are the differences? "TV B is better for this and this reason."
Ok, is there a reason to prefer TV A? "No, not really, TV B is better in every regard".
Also, prices in Dominion work real weird, since you cannot usually save money and often cannot make multiple purchases. It's like buying a TV while knowing that when you get home your partner is going to take all your money from you and spend it all on booze. Might as well get a nice TV then.
The TV maker, at the same time, knows that selling TV A and TV B at the same price would be a disaster, but that's not something the buyer ever thinks about.