Price is actually very much about the morph. For instance, creamsicles and red harlequins are morphs which are more expensive than tigers or patternless cresteds. Harlequin morphs are usually more expensive than flames of the same color, because harlequins have more patterning. A bright red patternless gecko is more than a plain brown gecko because of the color. Genetics are somewhat involved in pricing, but a gecko that does not display the coloration and pattern of its high-end parents (for instance a plain brown gecko that had red harlequin parents) will not cost near as much as a gecko that does display the same traits.

Normal, buckskin (tan/brown) cresteds are the cheapest, but you can find tigers and flames for under $75 as unsexed hatchlings.

Yes, a 29 gallon aquarium will work for an adult, but whatever you get should be vertical if possible since cresteds are arboreal (they do make a 29 gallon vertical tank I believe, so you don't have to turn a horizontal one on its end).