I need a car for taking my kids places evenings and weekends, so I can't get rid of it. It's paid for and (knock knock) doesn't need much maintenance. So my calculation is really only the gas I don't buy + parking.
My commute would be a 23-mile drive each way, 46 total. My paid-for car gets about 18mph, so that's 2 1/2 gallons of $4 gas. So that's $10 per day of gas I'm not using. And it's about $5 to park.
So I'm saving $15/day or $300 per month by riding.
And that, my friends, is what I buy whatever I want when it comes to bike accessories and clothing.