One possible calculation:
IRS mileage rate: 58 cents per mile, to cover vehicle costs (maintenance, fuel, depreciation, insurance)
Average speed 20 mph, so time to drive 1 mile is (1 hour)/(20 miles) = 0.05 hour
Desired profit per hour is $30 per hour so $30 × 0.05 hour = $1.50 per mile
Add the maintenance, 1.50 + 0.58 = 2.08 per mile.
Decide if you want to estimate the mileage for each job, or just use the distance of your average trip for everybody.