I have 3 charges when doing mobile... so far it's worked fine
1) $4/gallon @ 20mpg... so if I'm driving 20 miles, it would be a $4 charge... in your case 50 miles would be $10
2) travel time @ an hourly rate of $30... so if I drive for 30 mins, charge is $15
3) $15 load/unload supplies... considering it takes about 30 mins to pack up everything into little bins, put it in the car, etc.
Just remember to charge double for #1 & #2, since you're going to drive back home as well
