I'd value the cost of walking as negative: it's good for your health.
In our case, it did: we spent six weeks in LA, and we paid a good $500 extra to live in a townhouse that had a grocery store, mall and park within walking distance. What we spent extra on the location (I think) we made up in not having to pay for a long term car rental.
If we were doing this long term and had to weigh against buying a car, I think the math would have worked out differently.