You don't have to wrap the whole trailer. I do a wrap for the picture (logo) and use vinyl lettering for the rest. It cost about $1500/16 ft. trailer.
The question that matters most to me is not the cost, but the value to me. Yes, the trailer is a moving billboard, but how much business does it generate. Last year someone saw the crew at a gas station and call and I got a %5000 (gross) job from it. So I am having my 2nd trailer done. I think it is a good thing to do.
Just remember there is additional expense. You must keep the trailer looking good or it is a negative advertisement for you. It has to be kept washed and clean. My guys hit the fenders all the time. I have to keep that repaired.