You have to define what you expect to be able to get for a seriously old, 2nd hand processor though, considering I saw a 1.7GHz chip being auctioned off on eBay for 31 bucks.
Let's barter a little - as you seem to live in belgium shipping won't be too cumbersome either.
What heatsink are you using on it? If it's the intel original sink with folded alu sheet metal fins soldered to a copper base, I am interested in that one too. Or if someone else has a sink like that, it doesn't matter.
No way, a 256KB L2 400MHz FSB P4, is much slower than a Celeron 320 (2.4HGz 256KB L2 533MHz FSB) and a brand new retail Celeron 320 is only $78 (and the Celeron is even overclockable the s423 P4 is not).
A fair price for an old s423 P4 is more like $50-60 at the most.