1: 68-78 degrees
2: Yes, but I recommend turning the tank vertically since they are arboreal. This is the minimum size you can do for a single adult.
3: 50-60%
4: It just depends on your preference and use of lighting. Fake plants are easier and don't require special lighting so that is what I use.
5: Both--keep a shallow water dish and mist once or twice daily or as needed to maintain humidity (letting the cage dry out between each misting to avoid mold growth).
6: Price varies depending on morph, size and sex, but an unsexed normal buckskin should be around $40-50 depending on where you get it

Also, as noted make sure to feed the Repashy Crested Gecko Diet as it is the best complete diet for cresteds. If you feed insects you can feed crickets, phoenix worms, silkworms and/or feeder roaches a maximum of twice per week (make sure to dust with calcium and vitamin powder) alternating with the CGD.