A buddy of mine got a complete non-working Millipede at an auction for $200, spent lots time cleaning and repairing it, then only got $300 when he sold it on Ebay.
Also saw a complete Millipede with monitor issues on CL for $170.
These Millipedes don't seem to be commanding a very high price lately that I have seen.

It all really depends on who wants it and how much they are willing to spend on it.
If it was me, then $150 tops for non-working, non-restored.
For a working, average condition, $250.
For a working, great condition, $400, and that is if I'm really really wanting one.
For a working, restored, awesome condition... That's up to you... taking into account that you could easily spend $500 or more restoring one, plus labor and heartache

which usually accompany many of these restoration projects.
It is really up to you and how much you think it is worth to you. Good luck.
