I too work from home, however although cheaper than two properties, I pay for and maintain a property which is far bigger, and of a higher standard that I would if I was not working from it. As a result of the size it also eats energy. I have a separate bedroom which I buy high quality bedding for. I also have invested in a lot of extras, security system, bits to make the place look even better, and also have internet extenders in each room.
What I'm trying to say is that working from home still comes with costs.
Outcalls do however involve travel time, and cost, so unless it's an outcall right on my doorstep, I do charge extra. I also have minimum booking requirements for different locations, dependent on the actual direct travel time. This I find suits me and the area much better than trying to work out a mileage rate.