It all depends on what you need/want...
A brand new 18' dovetail with tandem axles and brakes can run around $2,000, if not a little more.
A good used trailer can be had for half that or less, if you look hard and find a nice one.
I picked up my 16' trailer, dovetail, with tandem axles and brakes used for $1000, needing some work. I've put about $600 into it in new tires, brakes, wiring, etc...
Ryan
