I totally agree with Debi above. When you’re trying to figure out your actual “take home pay” amount from mobile training, in comparing it to your in-gym training you have to look at some of the obvious expenses: gas, etc. along with some of the hidden costs: such as “opportunity lost” due to travel. A lot of trainers focus only on the dollar amount being charged/collected instead of factoring in the lost time due to travel. So, for example, a $100/hour mobile rate may initially look great compared to a $60/hour rate at the gym (these are just examples :-)), but if your total time (training and travel) to the mobile client took you 1.5-2 hours total, your actual per hour rate is lower, AND when you add in the direct costs of the mobile travel, that $100 rate is not so great all of a sudden.
I think that your question is a great one, and hits on the type of analysis a trainer must take a hard look at before deciding the rate they should charge for their mobile training practice. Take a look at ALL related costs before setting your mobile training rates!
I hope this helps.