Well, I bought mine used so I don't know how accurate this is but what the heck:
Cost of bike: $500 + about $150 in misc. stuff that I added
2004 miles: 1866
2005 miles so far: 950

(1866 + 950) / (500 + 150) = 4.3 miles : 1 dollar

So far, anyway. I have no plans to buy another one, and a goal of 5000 miles for 2005 which, if successful, will bring me to 10.5 miles to the dollar. I'm on pace to do a lot better than that

The original owner put probably 3000 on it. No idea how much he paid for it originally.