Are you talking about the Harbor Freight folding trailer? If so it is hard to beat that price point. You could do extensive modifications on that and still end up with it being cheaper than just about any other option. I went with the non folding trailer to get the 12" wheels. On sale now for $360
1720 Lb. Capacity Super Duty 48" x 96" Utility Trailer with 12" Five Lug Wheels and Tires
The wheels on these trailers do stick up above the deck, so you you are going to have to build it up a bit anyway. I used some 4"x6" as cross pieces and ran them out to 7' wide and then extended the tongue by 2 feet. I used it like that for several years with no problems. I finally wanted something that looked a little more finished, so I added a plywood deck, some carpeting and a homemade roller on the back.
I use it to haul all of my boats, including my 18' Aire Cougar, which is one big boat. It does hang over the ends, but so far that hasn't been a problem for me. I keep it pumped firm and it does fine. Having said that, I live on the east coast, so I don't have deal with the pressure changes that come from going over the high mountain passes out west.
I was able to get mine for only $275 at the time and it came to about $165 for the additions, for a total of $440. It will be hard to beat that price on a new trailer, though you might get lucky with a used one off of craigslist.
Here it is with my 13' Wildcat on it.