A lot depends on what you can scrounge, if you change a light fitting, keep the old one, and you have your heat. Next a box, cooler or a few sheets of plywood and some glass for a window. Your thermostat will cost anywhere between 10-100, or you could use a dimmer switch. next a soap dish and sponge for humidity and your pretty much done. I have a lot of material at my disposal so I have played at making turners and different shapes of incubators and heat supplys. I think my next design will be one that if works I will offer as a flat pack, need to do some more R+D first. If your looking for wood I would recomend melomine and silocone seal the edges to stop damp getting into the wood and it's easy to wipe clean. If I drill a hold I make sure it's the same size as a syringe so I can line the hole with plastic and use the plunger as a plug if needed. The trays could also be foil ones so you can recycle them after the hatch. now all you need is some wire mesh that I have yet to find a free supply of. A PC fan can usually be found for free on really old PC's if you get the power supply you also have the 12v adapter, if not the older cell phone chargers would work, even better is using a powerpack that you charge the car (again only if you alreadyy have one, and power the light from the mains plug and the fan from the cigarette lighter and if the power fails you have a back up. If you make it with internal dimensions of around 16 inches you can add an auto turner later if you desire. I like the idea of heating the air and then moving the heated air around the eggs rather than expose the eggs to direct heat as that would encourage hot spots in the incubator, however this is jsut design preferance, either seems to work very well.
Let us know how you get on