I like them for purchase only because the missions are really tough.
Though does feel like they're a way under priced.
Maybe. It would be easy to house rule a higher price. But since you can only buy one a round, there is a natural limit.
I would say the price of crown cards is effectively a game timer. Lower price = quicker game; higher price = longer game. Of course, with the cards, you also forego the potential to become a runaway leader.
I wondered though, with the mission cards, are the benefits substantial enough to also speed up the game? Seems they might chivvy things along a bit.