You can make a pinhole camera out of damned near *any* camera -- get one of the Speedex Jr. folders that are on eBay all the time, remove the glass and put the pinhole behind the aperture, you'll get a FOLDING pinhole camera with a shutter (and likely not have to do anything to the bellows -- I've owned two, and had to fix one pinhole leak). Same goes for the lower end 6x9 cameras. If the bellows is shot, you can hack out all the folding mechanism and bellows, drill a suitable hole in the door, and mount the shutter directly into the door (light sealed and glued shut) to make a very-wide, still with a shutter...
But you should be able to get both a Clack and a Brownie or similar 120 size box for under $20 plus shipping.
And trust me -- until you have one that uses roll film of some kind, you'll never make as many pinhole images as you want. I've got a pinhole shutter for one of my plate cameras, and I've made a total of about 9-10 films, of 6 scenes, in the past 8 months. I've made three times that many in the past 6 weeks with 120 and 35 mm conversions.
Heck, for a start, just drill a hole in a body cap to fit your existing SLR (assuming you have one) and mount a pinhole in that...