A heating pad is more appropriate for this type of animal. 4 crickets a week is not NEARLY enough to maintain a healthy leopard gecko!!

I would offer it six every two days. Whatever is not eaten, remove after a few hours.

The moisture chamber should always stay moist.

Rather than worrying about what is cheapest, worry about what is most effective. If you have to wait a little longer before you get your gecko to make sure you have the best setup, then it's worth it. (I studied up and saved money for two years before I got a bird)