I figure it's sort of a get what you pay for kinda thing. Of course I've never thoroughly researched trailer tires either, so maybe there's more to it than what I know. I bought a new set of 6 plys last year for $265 mounted at a friend of mine's tire store for my 18 ft' all metal, dual 3500# axle, bumper pull. I just said give me the cheapest 6 plys ya got, and have been satisfied with them thus far. I got a bigger size than what was on it (think they are 225/75/15 and don't recall brand) in order to get the dovetail up a little higher off the ground.
I imagine you could buy the cheapest 8 or 10 plys (whichever one you decide on) and be satisfied with them. Mine will probably dry rot from age before I wear them out from road mileage. I figure that's the scenario for most unless the trailer also doubles use for work, hot shot runs, etc.