Ok, your shit is starting to sound good. I have a project coming up where your service would come very handy so I'll PM you when the time is right.
Would it be possible that when you test proxies, you could also check geo location? It's piece of cake with MaxMind - GeoLite Country | Open Source IP Address to Country Database
for example. I'm sure many of us who target several countries would appreciate this and it would add value in your offer. So think about it
Like machinecontrol said, quality is everything. I have personally built scripts that scrape hundreds of free proxy sites constantly but quality of those proxies are shit. 95% of them don't work at all, ever. I have now been using some private proxy lists that costs little less than machinecontrol's but those have their own issues. Like for example you can't really do anything where your proxy provider might get abuse emails because they will close your account and there goes your good & fast proxies.