You need to show proof that your service works, because I have a lot more faith in pg than in you. If it turns out that you actually can get something to the front page though, it's worth a lot more than .5BTC imho.
Not sure how I'd prove this publicly (nor is it something I'd like "out there"). PM me and I can send a few upvotes your way.
It's got nothing to do with pg, really. Not sure how anyone would stop a voting ring. Most people get caught for doing stupid shit, like upvoting one link 50 times in a minute or using accounts that have no activity at all except upvoting that one link. My servers are constantly requesting the front page, pretending to read comments, upvoting links, and just generally acting like regular users. When I feel like commenting on HN, I query my main server, which picks a random account to use, so over time most of them seem fairly real. About half of them submit links too.
One thing I'm scared of is that the accounts are all fairly new, so that could be a way of filtering/catching them. But they're not
that new, and in any case it's not like I'm using them all at once. Thirty votes is usually enough to get you on the front page without raising suspicion because as long as your link is mildly interesting, being on the frontpage has this compound effect, and you'll get other upvotes that get mixed in with mine.
Back to my question though - how much would you pay?
Edit: Also, it's almost zero risk for the buyer. The payment system works so that my servers will upvote your link automatically as you send money to an address. You can buy a few votes, watch it go up, then buy more.