If your not in a hurry or you just want to build one from scratch yourself I would look around on Craigslist or ebay and see if you can find one for sale.
I am surprised at how many I have seen for sale that have already been started and or finished.
In fact I may sell my 7x16 because I just bought a 2010 29 foot fusion toy hauler because I got a great price on it,
http://www.youtube.com/watch?v=kGZy9mz_FUc http://www.youtube.com/watch?v=peo2sXp5NuYthis is the toyhauler I have now.
I had thought about building a longer trailer and it would have cost as much to build the trailer I wanted as this one cost me>?
I think after I built my trailer I found out there were some things I wanted like a better bathroom and RV toilet with holding tanks, also a onboard generator so I wouldn't have set the generator up and take it down every time, I should have put in more windows and built the Window AC in the wall plus about 5feet longer so I could stay inside without having to take the golf cart out.
I have never thought I would stay in a Walmart parking lot overnight but with what I have now I couldn't do that.
So I have something now that has it all.
I think the biggest problem I ran into was most trailer are not 6.6 feet high, and I wanted a 7ft for my golf cart so I ordered mine so I don't think there is any price break if you order one, but you can get it the way you want.
this is a great web site to look around and compare prices
http://www.trailershopper.com//urlI live about 100 miles from Douglas GA and trailer building is the main industry and has been for many years, I talked to a guy that works near there and he said they build 450 trailers a day.?
and they can't keep up with the demand.