I would budget something like this:
(prices in GBP)
Vertex, Timor, Record, Cyma, Buren: 200-250
Lemania: 300
Eterna: 400
Omega: 400-500 (don't know why, they are quite common and 25000 were delivered. Many redials out there)
JLC: 850
Longines: 1000
IWC: 2000
Grana: a seller could name a price! Won't be less than 2500
The Buren is not all that common, but still goes for low prices. Ebay prices do sometimes seem all over the place and bear no relationship to the scarcity of each brand (other than the Grana). You may find bargains out there, but they do seem rarer now.
Good luck with the search. I'm glad I got mine when I did: