It might not be worth that much, especially if you plan on replacing most of the components. However, miyata made a wide range of frames; some quality, some crap. For example, I converted a 512, which is a decent frame they produced in the 80's with tight geometry and triple butted tubing (I think). I got that in a trade, but I might have paid $200 for it.

For the most part, old japanese frames are going to have pretty standard sizing, threading, etc. Very easy to convert.