TLRs and folders are your best bet for sub-$50 MF cameras. With one or two exceptions (which will almost certainly exceed your $50 budget), though, these cameras have permanently mounted lenses, and for $50 you're unlikely to get a built-in light meter.
If you want an inexpensive MF camera with interchangeable lenses, the most likely prospect is a Kiev 6C or 60. This camera looks like a typical 35mm SLR, but it's bigger. It was also made in the former Soviet Union, in Kiev Ukraine, and it's not exactly renowned for its reliability. You can find them on eBay for about $100-$250, shipped. (Many sellers are located in Eastern Europe, and the shipping is likely to be $50 or so, since these are heavy cameras.) If you keep your eyes open you might find one from a domestic seller for $50, but its quality would be suspect. If you're willing to take a chance, tinker with the camera, and deal with its quirks, a Kiev may be a serviceable option, but I'd recommend a TLR over a Kiev if you want something that works reliably. Note that some people say the Kiev 6C/60 is a clone of the East German Pentacon Six. This isn't quite true, but they are similar designs that use the same lens mount. The Pentacon Six tends to sell for a bit more than the Kiev 6C/60, but if you find a Pentacon Six for a good price, I'd put it in the same general category as the Kiev 6C/60
Another low-cost option would be a Mamiya C-series camera. These are TLRs that take interchangeable lenses. These sometimes sell on eBay for as little as $100, but $200 and up seems more common. By all accounts these are excellent cameras. (I've been wanting one myself for a while, but haven't yet set aside the money to buy one.) They're probably a better choice than a Kiev 6C/60, but they usually sell for more on eBay.