To Mr. & Mrs. Spock,
Before I say this, I want to say I admire the folks who have been able to successfully make their own incubators, and there are several on here. It is not my intent to belittle these endeavors or suggestions. And if you folks are interested in building and experimenting, you may well have good luck building your own successfull incubator. That said, since you have said that you are on a fixed income, and cost is an important factor to you, I would suggest, as someone stated above, the Little Giant Still Air incubator, which can probably be bought at your local farm store for $40 or $50. It is what I use, fairly successfully. You can double the cost by getting the fan for it, which stirrs the air around inside, but I don't have the fan, and you can triple the cost by also getting the egg turner, but you can mark an X on one side of the egg, and an O on the other side, with a pencil, and turn them 3 times a day yourself, since you are retired and probably home rather than working out. I don't have a turner, either.
Now, for building your own, by the time you buy the porclain light socket, the dimmer, and whatever other things needed, you will also have spent some money, and you will also not have anything accurate till you work and experiment with it, while the store-bought starfoam incubator will be pretty well ready to go, which would be a good thing in my opinion, since you are just starting with this hobby.
These are just my personal opinion, and not meant to step on any toes of those who have successfully built their own incubators.