Good point.

Actually, I wouldn't even factor in depreciation (or leasing costs, in the case of manufacturer lease program cars) on the car, since the car continues to depreciate even if it's sitting on the lot. I suppose you could argue that its value decreases faster as more miles are put on it, though, so I'll factor that in.

My back-of-the-napkin math for variable costs shows this:

Detailing
Standard 3.3 cars cleaned per hour = 0.3 hours x (($10/hour x 125% approximate tax/insurance) + $2/hour benefits) = $4.35 in detailer labor per car cleaned
$0.50 in chemicals and equipment maintenance per car cleaned
Total: $4.85 per car

Rental counter
Standard six-minute transaction = 0.1 hours x (($10/hour x 125% approximate tax/insurance) + $2/hour benefits) = $1.45 in rental sales agent representative labor per car rented

Depreciation attributable directly to mileage
Average 75 miles per day x average three-day rental x mileage depreciation rate of four cents per mile (cite) = $9 per transaction.

Preventative maintenance
3 cars per hour = 33% x (($15/hour x 125% approximate tax/insurance) + $2/hour benefits) = $6.85 in labor per PM
4.5 quarts x $1.15 per quart = $5.18 in materials per PM
Total cost per PM: $12.03
Average 225 miles per transaction / 6,000-mile maintenance interval = 3.75% of maintenance cost x parts and labor cost of preventative maintenance (LOFR): $0.45

Total variable cost per average three-day, 225-mile rental transaction: $15.75

So, assuming the car would otherwise be sitting on the lot and that staffing could be reduced without these rentals:
If Hertz can make at least $15.75 on the transaction, they're even. For a three-day rental, that's an average of $5.25 per day.

Kind of a random post, but I was curious what it would come out to. It's a good figure to have handy...