Proline and Kaufman trailers are the same. I was vey interested in their 35 ton detach advertised brand new at around $28,500. After it was all said and done it was well over $40K. So I just bought a used Challenger 35t. That price was for spring ride not air ride. Price didn't include pony motor, boards in the middle, beavertail, wheel covers, aluminum wheels, or that monster F.E.T tax. Their advertising was good with the pictures of what was an equipped trailer with a bottom price. I still think it is a good looking trailer but I know the steel isn't as thick as the big guys. Still cheaper than a used Eager Beaver almost.
Just call them and get the actual out the door price with your options. That is a cool story in that link and he has some nice taste in trucks I'll say. One thing I don't like about that tag trailer is the C channel beams on the outside are turned around compared to an Eager Beaver and most every trailer I've seen (unless those are I beams but I doubt it and that would force the boards in even more). That means you can't hook a chain underneath and catch a cross beam to fit different machine sizes. It seems D-rings aren't always where you need them to be. They say they aren't cheap but affordable but I'll bet the steel is thinner where it can be.
I bought a used Eager Beaver tag trailer and really like it too. They have good resale. Not sure about a used Proline.
