I agree that there must be an actuarial way of measuring these things. I would also argue that the number should be easy to find and work with. After all, if it takes 2 days working with a stats package and actuarial tables your colleague will literally never look at it again.
I think the IRS's business mileage allowance rates might be a reasonable compromise. It can't be too far out either way because otherwise, no one would use it if it's too low or they're giving away free money if it's too high.
It is calculated using "an annual study of the fixed and variable costs of operating an automobile, including depreciation, insurance, repairs, tires, maintenance, gas and oil".
For 2018 it is 54.5 cents per mile. For the rest of the calculation, I'm just going to use the numbers for an 'average' American, of course, you'll be able to plug your colleague's actual numbers in.
Annual allowance = total miles * $0.545 = 13,500 * $0.545 = $7,357.50
Less insurance = annual allowance - insurance = 7,357.50 - 750.00 = $6,607.50
Divide by 52 to get weekly = 6857.50 / 52 = $127.07
That still includes gas, so subtract weekly gas, say $30 = $97.07.
I'm in the UK, so I'm afraid I have no real idea of how much gas and insurance will cost in the US so I dropped in numbers that feel right, but you'll have the actual numbers you can put in.
The first hidden assumption in these numbers is that maintenance of an older car will roughly balance the depreciation of a newer car. A real banger or lemon could easily require much more maintenance than average.
The other assumption I see in this calculation is that the cost of insuring a car that isn't driven much could outweigh the amount of the allowance for the milage, but I'm assuming that all 3 cars are covering a reasonable distance. If one is a rarely driven pleasure car then it might be best to calculate the amount for the other two cars and add the weekly cost of the insurance onto the top because its other expenses are likely to be quite low.
It doesn't look like $100 per month is that far out for an average though.