Reddit opens self-serve ads to the public (promo your site for as little as $20)
blog.reddit.com
blog.reddit.com
I had some problems with their credit card processor and they (being reddit) responded to my emails withing a few minutes until we were both sure that everything was as it should be (this was impressive to me because I was spending such a small amount of money).
That and it ended up costing me about $0.10 CPC - considerably less than I remember spending on google ads back when I tried them. The lottery system really scared me, but it ended up working out pretty well. I'd definitely recommend it to anybody else.
People say that redditors are a terrible market to advertise to (they use adblock, etc.) but I disagree. I think they'd probably be a terrible audience to market to if you're selling teeth whitener or pyramid scheme "Google pays me $1000/hr from home!" things, but if you've got something useful to show them (like a webapp that they would like) then you're going to be buying a lot of eyeballs for not a lot of money.
All-in-all try it out - I was really happy with it.
It's a sort of interesting idea, but completely impossible to calculate ROI etc before you spend.
Also, WTH "only accepting US credit cards." Hate it when sites pull that one.
The projections for the ammount of money that will be spent [in/on] this little endeavor are astounding, to say the least.
For a cool new consumer product or service that makes money directly, though, this could be great. When I tried it, the CPC was very low. Whether it's worth it depends on how good a fit the thing you want to advertise is with the audience of Reddit.
I can't imagine making a great return selling something directly on there unless you product was so cool that a regular post would have had nearly the same effect.
(edit: seems to have been fixed now)
It will go exactly as high as people are willing to pay for it.