The price you get on the tag depends on what contract the trailer was bought under. The tag will be the result of the total contract price divided by the number of trailers (in this case). But one contract total can be very different from another.
As an example, with made up numbers for simplicity, if a contract is let for 10 trailers and nothing else, and the total is $20,000, then the trailers cost $2000 each and that is what would be on the tag.
But if the contract was for 10 trailers, plus misc. parts, plus two spare tires for each trailer, and training soldiers on maintenance of the new trailers,the total contract might be $30,000. Since there are still only 10 trailers, the total will be divided by 10 and the tag will say that each trailer cost $3000, even though the actual trailers only cost $2000 each.
Works pretty much the same for everything the government buys. Makes sense to someone, I suppose.
Cheers