Incubator, Homemade VS Buying ????

How would you make a good quality homemade incubator, and how much would it cost to have everything customized and set up with it (Heat source, thermostat or thermometer, etc)

How much would a good size incubator cost? I only have 2 ball pythons atm but if I get into breeding I'm obviously going to get more. Should I start off with a small one or go straight to a big one?

And I have a question about the "substrate" the eggs have to be on. What do most people use, how do you make it, and what am I looking at for cost?