First, storage doesn't cost google all that much - on the app engine they charge $0.24/GB/month, so their own cost for gmail is likely a lot lower than that. And it's hardly 10GB per person - I'm a heavy user with lots of email, never delete anything, etc. and I have 0.8GB in my gmail this very minute (compressed or not? I'm not sure). So for their average user, probably less than half a gig. So maybe $0.10 a month for the storage. There are other costs, to be sure, but there's a ballpark.

So a couple ad clicks a month would probably do it - and you forget the biggest thing - gmail means you're that much closer to google. They have more info about you, you'll search through them more, you're more likely to get an android phone, etc. etc.

Gmail isn't going away.