I'll second the harbor freight idea but would add that you can even buy one of thier $200 utility type trailers for a 12' jon. IT's light weight and flat bottom will let that boat ride on almost anything. If buying used, you should be able to find something in teh $100-$150 range easy enough.
p.s. If you have a SUV or PU, just get one of those bed extenders (also from Harbor Freight) and you can slide it right in the back without bothering with a trailer at all.
that is a long PWC trailer.