For a blank one, you would be looking at 250-300USD.
As for one with data, crap games go for about 300USD and something interesting pushes the price to the 400USD zone.
I think 300-350 EUR for one with data, 250 EUR for a blank one.
http://www.assemblergames.com/forums/showthread.php?29991-Jackheads-Partner-N64-Kit N64 Devstuff
http://www.assemblergames.com/forums...station-Dev-PC PS Dev setup
Untested ones are about $150-200 here in the states. I sold one over on digitpress a few years ago and they haven't changed much since.