I think the first thing you need to do is get anyone interested in going to vote on where they'd like it to be, as costs will vary wildly from country to country.
You'd have to stick to big cities in order to allow for easy / cheap flights and to remain easily accessible by Rail & Road.
London would be ridiculously expensive, but Birmingham, Bristol, Cardiff or Leeds would all be reasonable.
Paris, again is very expensive, but there's places outside of Paris that would be reasonable.
Somewhere like Prague would be good cos its nice and cheap there (<£1 a pint tee hee) and it's fairly central in Europe.
Amsterdam would also be good (again very easy to get to, plus lots of fun in the evenings)
Once you've decided on a City to go to, you can easily figure out all the other stuff, such as Hotels, Venue, transfers etc etc.
Just my 2 Penneth
If it's in Amsterdam, I may just take a long weekend myself, just to come over and buy the Developers a thank you drink, after you've all done your Guru thing at the venue