UTHs are $15-20 I think (you want one that covers 1/3 of the bottom of the tank), then a tstat is about $25. The sphagnum moss idea will also help, or using a substrate that holds humidity better (like cypress mulch, newspaper, or Repti bark). But if you always use a heat lamp, you'll be constantly fighting to keep the humidty up. I'm talking misting the tank multiple times a day, which can cause bacterial growth.

In the long run it's just easier to use a UTH, especially since BPs do well with belly heating.

I think anything you do is going to require some investment, the amount will depend on what you choose.