Yup. You could do it all for under $30 no problem. You can usually find styrofoam fish-shipping boxes for free from pet stores. I work at a big chain pet store (not petco) and we will give away the boxes that our feeder fish come in. The ones that the rest of our fish come in we send back to our distribution center, but I believe it's the same with most of our stores and we get them every friday evening. bricks are a couple bucks at a hardware store, the egg container can be a shoebox sized rubbermaid from wall-mart for less than $2. Use perlite or vermiculite (also found at a hardware store) for the egg substrate.The submersible heater, 25 watt max, you should be able to find for under $20. A good analog thermometer should work fine, but a handy investment is a digital indoor/outdoor thermometer with a probe that you can stick inside the incubator so you can keep a constant eye on the temp. without opening it all the time. You should be able to make your money back on all of this by selling one baby.
There are a few different ways to build these incubators. Check out the other threads about these and I know there are some websites with instructions as well.
I know you were looking for a python. Did you end up choosing a corn snake instead or are you getting both? Good luck with whatever you end up breeding. Let us know how it goes.