I've been toying with making my business trip charge
dependant on gasoline. For example, choose a gas
station. My trip charge is 20 x G, with G being the
price of galon of gasoline. Labor rate is also 20 x G.
I'd have to figure out what's fair. But, the cost of
service would go up and down with gasoline.

Has anyone does this? Does it make sense?

Yes, no, absolutely, I don't know, and maybe. Some businesses charge based
on mileage if their clients are different distances from the service point.
Others roll all their calls together, average them, then set the price for a
service call. It is dependent on your business. If your business is one
where you can go to the client's home, perform the job with minimal running
around time, and perhaps have a stocked truck to guarantee that, then you
would go with service call plus labor, or some compilation thereof. If the
job consists of going to the client, going to get parts or to fabricate the
job, and return, then mileage would have to figure in there somewhere unless
your total price covers it.

I drove to Las Vegas yesterday, a total of 135 miles from my house. I drove
about 40 miles in Vegas, then drove back. I left at 8AM, and got home at 6
PM. I made right at $800 for four hours work time there. My total day was
10 hours, but there were other personal errands conducted during that time,
including an in-office procedure at my doctor's. I do not charge my boss
mileage because the hourly figures out to $60 an hour or a lot more.

You just have to average in the variables, decide if your market can bear
it, see what the competition is doing, and adjust accordingly.

Steve