It's not about the money
Biking and walking can save you money. Sometimes a lot of money. But I won't go into the enormous amount of money you save. Nor will I expound upon the impact you will have on government spending or on benefits to the local economy.
For example, I will not point out how much cheaper a bike or good shoes are compared to a car. Simply looking at the sticker price of a new bike—even a top-of-the-line model—shows you that you could have several bikes for the price of a car, new or used. (The cheapest car is the Nissan Versa starting at $11,990; the kind of bike to ride to work is well under $1,000 and most are closer to $100 if you buy used.)
I also won't tell you about the low cost of wear-and-tear on a bike or shoes. It's probably fairly common knowledge that a bike inner tube is less that $20 and new tires can run about $50. You can compare that on your own to the hundreds you spend on tires for your car.
The gasoline savings are so obvious that even a child could calculate them, so I won't bother to say anything here. (By the way, you save 100% on gasoline by walking or biking.)
Something that you might think of but I absolutely refuse to bring up is the lower cost of maintaining infrastructure. Why would I want to talk about saving any level of government the money that can be spent on repaving roads? It's not like having the streets torn up from constant pounding by giant machines inconveniences anyone.
Finally I will not bore you with the secondary and tertiary benefits of having a walkable and bikeable community. The fact that a place with walkable and bikeable streets has a more vibrant local economy is something explained in such detail by the mainstream media, that I would not wish to be redundant. Just click here!