[furchin]

This is absolutely possible but your numbers are off by a factor of 10, which means it'll be prohibitively expensive for many folks. I worked in advertising once upon a time; do you know the demographic that advertisers are most interested in reaching? It's not women 35-60. It's not men 18-15. It's not college educated minorities in Alabama. It's not any of these. The folks advertisers are dying to reach -- the reason they spend all that sweet, sweet ad money in the first place is to reach people who have the discretionary income to throw away $5/mo on a free website. People who spend money on the internet -- for a site that is free no less -- are going to be a gold-mine. The average ad revenue per user of this demographic is easily 10x higher than the overall per-user ad revenue site-wide; and if you take away these users advertisers will pull back spending and you'll actually get a bigger loss. You need to charge way more in subscription fees to make up for this ad spend loss.