I think that every farmer has what I've heard called the "line of shame". It consists of equipment that you thought would make life easier in one way or another, but for some reason just didn't ever work out. That new planter, or seeder, or... well, just look around at any farm and you'll find the line.
Dump trailers in this area run between $3k and $7k, the bigger the trailer the higher the price. People don't sell used dump trailers very much; I looked for a while (weeks) for a used one, but found that the discount for used wasn't much, and given than I'm using this trailer to feed animals, I need it to work, so I bought new.
Figure it'll pay for itself in 3-4 months.