There is no honest single number, because a cargo van and a tanker are not the same job. What a quote actually turns on is six variables, and knowing them lets you compare two bids that look different on paper.
The six variables
Unit count and type
A tractor-trailer is roughly three times the surface area of a cargo van and takes proportionally longer. Mixed fleets are usually priced per unit type rather than as a blended rate.
Frequency
Recurring work prices materially below one-off work, and the gap widens with volume. This is not a discount for loyalty — a fleet on a monthly cycle genuinely takes less labour per unit because the soil never fully sets.
Condition
A fleet washed monthly costs less per unit than one washed once a year. The first wash on a neglected fleet is often quoted separately as a reset.
Scope
Exterior wash, trailer washout, engine degreasing, aluminum brightening and detailing are separate line items. A bid that looks cheap is often quoting fewer of them.
Site access
Gate windows, plant badging, confined work areas and overnight scheduling all affect crew time. A yard the crew can set up in and work continuously is cheaper than one with a two-hour access window.
Location
Everything inside the 100-mile radius is priced the same way. Beyond it, mobilisation is quoted separately.
When a contract beats per-visit pricing
Roughly ten units on a recurring cycle is where contract pricing starts to win. Below that, one-off pricing is usually the better deal and a vendor who pushes a contract anyway is selling you something you do not need.
Above it, a contract buys three things beyond the rate: a standing slot on the schedule, priority ahead of one-off work, and a per-unit condition report on every visit — which is a dated record of what each unit looked like, and is worth having the day a damage claim shows up.
The line item most bids leave out
Wash water containment. A mobile wash operation without recovery equipment is cheaper because it does not own the equipment, and the cost it avoids reappears as your liability — the discharge happens on your property under your stormwater authorisation. If two bids differ significantly, ask both vendors where the water goes.