There are events of all sort of price range and no qualitative difference :- ). Some of the best CGI events (I am talking always 1-2 full days of presentations/workshops) I went to were around 100 +/- Euros per person or less.
The ones more corporate industry oriented (Siggraph for general graphics, Game Developer Conference for real-time, E3 ) do go for 500-2000 Euro often...
I think the ones for Archviz (SOA, D2, RenderIt, etc..) are well priced at +/- 250 Euro per person. If you are freelancer and you have one near you than with travel costs and accomodation it comes up to acceptable rate if you consider it as part-time vacation.
For me for example SOA is really the best because you can spend day in Venice, another in Bienalle and then go home making your money worth even if the presentations were bunch of marketing crap :- ).
But truth be told from someone who went to at least 15 events and spoke at half of them, you're not missing much. You can network and find inspiration in your local pub. I really envy more events like London Meetup and similar. And you can always make one like that yourself in your city.