It's not much money. $10/mo is $360 per machine if they replace the machines every three years. Plus $75 in search revenue ($25/yr) is $435. For $20/mo that total goes up to $795. That gets them up above the typical cost for a netbook, so they could start making money. But factor in maintenance costs, server overhead, etc... seems like pretty slim margins.

That said, if they can get large swaths of people using Google who previously couldn't afford a computer, they could be expanding their reach massively. So maybe it's worth it in that sense. To acquire virgin lifetime Google users.

Edit: I guess they're selling them with an upfront cost, I missed that. But it's an interesting calculation nonetheless.