Cresteds are pretty awesome - I own nine of them, and a Ball.
In a 40 you could keep two, and I would recommend sub-adult to adult females. One female with one male with result in the over-breeding and possible death of the female.
If you were to put hatchlings in a 40, they'd get lost! XD

Cost-wise, I've seen adult females for around $100, but they can be very expensive, some ranging up to $300 - $500. Males are cheaper, but don't ever keep too males together.

Food wise, CGD every other day and crickets twice a week keep mine pretty healthy.

Good luck!
~ Liz