I think that between $400 and $600 (US) is a fair price for a working head in good condition, at least in my area. Silly me bought a '71 V4B about 5 years ago for $200 locally and sold it a year later for $475 plus shipping. I've been searching for that combination of tone and mojo ever since.
On the flip side, I have a Pignose B100VH which is a no-frills 100W all-tube head that sounds surprisingly Ampegish and is less than half the weight of a V4B so don't forget to keep your options open a bit...
'82 AVRI '57 Precision | '87 AVRI '62 Jazz
Orange BT500 + Schroeder 1010