I am proud to say I spent about $1,800 on a kickass, custom-designed (by me), LBS-built commuter bike. I took ownership in late August 2008. Given that I would have to take the CTA if I didn't commute, I would be paying aobu $4.50 per work day (or $86/month depending on how you want to calculate). I conservatively estimate that I commute between 175-200 days per year (out of a possible 250 if you give 2 weeks vacation).
Based on those numbers, I think I have paid off my bike and view it as a profit center.
That said, I probably need another six to twelve months to amortize various accessories that I paid for myself and didn't get as birthday/Christmas gifts, as well as tuneups, a new chain, tires, and tubes.