50 pin drives are used on a number of vintage machines, so people bid them up. 68/SCA tend to go much cheaper (but the adapters and sca converters cost a few dollars).
I don't toss any of my working drives, the ones with major read errors get the controller board removed (for parts like capacitors, edge connectors, and RAM chips) and the rest junked. Everything I get is tested and put on the shelf for when I need it. Oddly enough smaller capacity IDE is getting harder to find too.