Hard to say. Trying to work it out did throw up the interesting figure that from 4 Dec 2004 to 24- Dec 2008 I did 9.989110422 miles per day on average!
Since I have a car, and 95% of my rides are for pleasure rather than replacing car trips, and I spent a mint on cycling stuff, I must be down cash-wise!
If I asssume that each utility mile cycled is 40p not spent on a car mile, and that 5% of my mileage was saving a car mile, then I have "saved" £295 over 4 years.
That is set off against perhaps 10-20 times that amount SPENT on bikes and kit!
I am over 3 stone lighter though!