You do NOT need an under the cage heat pad with crested geckos UNLESS the room you are keeping them stays cool all day/night year round. With room temps fluctuating between 70-80 you will probably overheat them with a pad. Also it is imperative they never be kept at 90. Anything over 85 will cause them to deteriorate very very quickly and die. High temperatures are cresteds Achilles heel. They also should not be kept at 85 for long. Your best bet is to never let them get over 82.
You need to look up breeder care sheets. If you type in google 'crested gecko care sheet' you can find any one of 25 care sheets that will all tell you exactly what I did about the temperatures and heating devices. I believe there is a care sheet here on GU with that very information as well. A plain hygrometer can be gotten for a couple of bucks but like I said before I got a very nice digital hygrometer/thermometer combo with a probe from an indoor gardening shop for $20. I know money means different things to different people but I am not sure I would consider that I big chunk of change.
Also if you are looking for high quality, young crested geckos for a very reasonable price, check out the breeder I use at
Welcome to the NEW ACReptiles.com. He offers a 3 gecko "grab bag." The down side is you do not get pictures ahead of time but he tries very hard to meet your request. I got a beautiful brindle, a harlequin and a fireback all very healthy and all with perfect tails for $149 including shipping which is tough to beat.
Welcome to the Anthony Caponetto Reptiles Online Store - Crested Geckos :: Crested Gecko Package Deals :: Wholesale Grab Bag - 3 Pack - Perfect Tails
is a link to the actual grab bag page. I surfed a lot of breeders before I was recommended to check this guy out by one of breeder suppliers for researchers at my school. He is the best I could find
on a budget and one month later I am still very happy with my purchase. Lots of info about crested care and such on his site as well if you are looking for an excellent care sheet.