Cheap digital backs would probably be tricky.
Presumably, the customer would want one because it would be compatible with an existing SLR and lenses, and allow them to learn and use the same shooting techniques.
Unfortunately, 35mm silicon sensors are Not Cheap, so anything that is cheap is a lot smaller. Sensor processes have gotten pretty good, so cameras with nasty little sensors can wring out surprisingly good pictures; but that's with optics matched to their size. With an optics path expecting a 35mm area, it'll be like shooting through a straw. You might be able to put one or more intermediate lenses on top of the smaller sensor, to provide a 35mm-size target and re-focus the image onto the actual sensor; but doing that is going to add distortion and eat some photons(the first you don't want if you are doing this to use your fancy SLR glass, the second you don't want because you are using a lousy, noisy, low-sensitivity sensor that really needs all the light it can get).
Because of those constraints, cheap SLR backs would likely be worse than cameras of equivalent price(especially since they'd have to come in multiple flavors to support different SLRs, while the cheapy point-and-shoots need be compatible with nothing but themselves and SD cards), which(along with actually-good DSLRs) confines the market for digital backs largely to crazed, price-insensitive, medium and large format devotees. Some pretty cool stuff; but for the price of a new car, you'd want it to be.
Doom and gloom aside, though, if one were willing to put up with the limitations and do a bit of bodging, I strongly suspect that you could get a functioning digital back, complete with handy screen and storage interface and things, by tearing the face off a poor, unsuspecting, digital point-and-shoot and then shoving it in such that its bare sensor is as close to where the film should be as possible.
You would have to contend with any self-testing that the camera does that might alert it to your alterations(spoofing of anything you have to remove or CHDK wizardry might do the trick, really ghastly cameras probably won't even check); but modern cheapy digitals are quite compact indeed, so finding one that fits should be doable, and the process of stripping a camera down is usually pretty simple. Whether you 3d-print a nice mount or use copious electrical tape to keep light out would then be between you and your conscience.
Plus, with ghastly point-and-shoots practically being given away(doubly so for used ones, or ones with damaged optics that you would be stripping anyway), it doesn't hurt to break a couple during the course of the R&D!