I would imagine a flatbed is just like a trailer, you just can't build one for what you can buy one. Unless it is a custom bed / trailer.
I built a 14-feet flatbed dump for a 2004 F-550 I had, don't remember how much money I had in it. I sold the F-550 in 2007 and bought a 1-Ton Dodge. I had every intension in the world of building a aluminum flatbed for it. When I started pricing aluminum, just couldn't do it. I bought one of these.http://www.alumaklm.com/truck-beds/item/73-9000-truck-beds.html