They were likely pricing per trailer foot. Not sq ft. I charge $175 per trailer foot. Ex: 17'=$2975 plus materials (which can be between $200-$500). That price is just for the actual polishing procedure, and notstripping or repairing anything.
Also depends on the overall condition of the surface. I have polished good condition trailer with great results, but also poor condition trailers with ok/shiny but scarred exterior results.
Each day you polish, you will finish up resembling a coal miner clocking out. It really is a very labor intensive, and grungy job, hence the price. No one REALLY wants to do it, but people have to make money and fill the demand, so alot of shops will price it accordingly depending on condition.
I just returned from an inspection/estimate for a Caravel
and the estimate was about $4500 because of the condition, and the clear stripping.