I'd be generous in saying that my car gets 25 mpg on my commute, but it sure makes the math easy. I've saved 40 gallons of gas (a little over 3 tanks).  At an average cost of $2/gallon, I've saved nearly $80 in 4 months of biking.

To recoup the entire $2000 investment in the bike (since I'm excluding maintenance savings in the car, I'm also excluding maintenance costs for the bike), I'll need to go 10,000 miles at today's gas prices.  So I'm on pace for a payback of between 3 and 4 years.  Or gas could go back up to $4/gallon again.  That'd be... nice?