Reuters says that Fon will charge $5/€5 plus shipping and tax for a Buffalo or Linksys router: In exchange, the Fonero must remain on the phone network for 12 months. The Fon network relies on principally wired backhaul purchased separately by the Fonero, and expects the Fonero to observe local network limitations, but doesn't enforce ISP no-sharing policies. They're working to sign more ISP partners who will appreciate the line-land-to-hot-spot notion. (They're charging the same unit value in euros or dollars for all their services, it looks like.)
Fonero who share their connections at no cost to other Foneros can use all Fon locations at no cost. Those that charge a day fee (about $3/€3, intended to discourage non-Fonero use of Fon spots as a replacement for an ISP connection) have no free roaming privileges, but their locations may still be used by no-fee Foneros for no fee.
One million routers sold for $5/€5, even with a great purchase deal, could translate to $20m. And they just raised slightly more than that a few months ago. Since there's no revenue pipe as of yet, this is tricky math until they explain more of the details.
Update: A commenter notes below that the billing system, noted at the end of the Reuters article, will provide revenue. I'm still not confident of the revenue side of Fon given the emphasis on and clever marketing of the Fonero-to-Fonero connection. Still, at $3/€3 a day per occasional user with a million routers, Fon's take could be sizable if the idea has legs.