Costs really differ depending on things like location of the unit, the size of the room, if it's energy star rated, etc.
From a long time breeder of cresties: it ain't cheap. It's 100% necessary though.
Please please figure out how to keep them cool now! Don't wait until spring. If you can't, you need to give them up. There have been several breeders just last year that have lost entire collections due to heat. A/c units failing have killed thousands of dollars of cresteds all over the country and one breeder in Canada lost over 200 geckos because for the first time since they started breeding their temps hit 90 last summer. (Grr, climate change) They had no way of cooling and sadly many of their sticky toes died
I really urge you to see if you can financially afford to cool your cresteds. The bearded breeders think we get off easy because we don't buy UVA/UVB bulbs left and right, but we make up for it keeping our stickies cool in the summer.