I'm using v3exchange too, coz they seem to have a lot of people in their network.
My click through % is around 0.25%, that's 1 click every 400 impressions, but then I'm only displaying my banner on the most relevant sites.
I'm displaying approximately 10,000 impressions a day to receive 0.25 %... overall, after 503,393 impressions, 1165 have clicked through (0.23%). Don't know how many of them are customers though.
At this day and age, I don't think banner advertising is the best way to promote your website. I use it more to generate an awareness instead of to generate sales/traffic...
I'm pretty sure that most banner exchanges are the same.