There are a lot of mechanically sound used 870s out there in used gun racks and gun shows. Been awhile since I looked for one, but used to be easy to find them for $300 or less., and the police turn-ins will have an 18-20" barrel already.

I stocked up a few years ago. The two on the left are 870s, the rest are assorted Ithacas:

[Linked Image]