I made a 4x5 pinhole camera this spring for WPPD. I already owned some 4x5 filmholders and shim stock, so my bigger expenses were a (lifetime supply of) brass blackening liquid and two T-nuts for horizontal and vertical tripod mounting. (The store was out of zinc plated nuts so I bought stainless!) A basement and garage/workshop full of miscellaneous miscellany supplied scraps of birch plywood and other material. I think it was some careful attention (and unusually good luck) with making the actual pinhole, but I believe this year's effort gave the best results I have ever gotten with a pinhole camera.
Anyway, it's fun stuff.