"350,- EUR ballpark"
I'm out!
Er, I don't know... maybe! If it ever gets done. 
If it's a plug and play board I'm probably in.... er, I don't know.... LOL
Geeeeeeeeze! Just get 'er done Jens!
350? Sign me up... the Alice 4000+ PCB cost me 100 Euro. The custom chips cost me over 100 Euro. The Video DAC and a couple of other components from Hese cost another 25 Euro, the SIMM sockets cost 16 Euros, all the passive components cost me nearly $200 USD from Mouser. And this is all not including shipping (most of it international -- read as "EXPENSIVE"). Couple that with the amount of time I am spending building it, 350 euro is a bargain.
Where it gets hairy is that you'll have to provide an accelerator card, so, plunk down another $300 with Jens for an 030....so, hopefully, you have one already, since it doesn't have a CPU on board.