The per-unit rate is the least interesting number in this comparison, because the largest cost in two of the three options never appears on an invoice.
The three options
Drive-through or truck stop bay
Lowest sticker price per unit. The hidden cost is downtime: a driver deadheading to a bay, queuing, washing and returning is an hour or more of paid time and fuel per unit, and it is time the unit is not earning. It is also a brush wash, which is what shortens decal and wrap life.
In-house wash pad
Lowest marginal cost once built, and the highest capital cost by a wide margin — pad, interceptor, water supply, permitting and ongoing discharge authorisation. It also converts washing into a labour management problem, which is where most in-house programmes quietly fail: it is the first job cut when the yard gets busy.
Mobile service
Highest sticker price per unit, zero downtime, no capital, and the water handling is the vendor's equipment rather than your permit. Works on a leased yard with no infrastructure, which is what most fleets actually operate from.
How to actually compare them
Add the driver hour and the fuel to the drive-through rate before comparing it to anything. For a tractor at loaded labour cost plus fuel, an hour of round trip frequently exceeds the wash itself, and it recurs every single visit.
For in-house, amortise the pad and interceptor over a realistic life and add the labour, the water and the permit administration. In-house wins at genuinely large scale on an owned yard — it is a poor answer for a twenty-unit fleet on a leased lot, which is the situation it is most often proposed for.