Complete I have just under $2000 in this one. The only equipment I needed was a welder and with BuzzSawyers help we had most of the main welding and support stuff done in a weekend.
I purchased the axles out of
http://www.etrailerpart.com/ along with the rims and tires. The axles were 7000lb axles with breaks and they were about $300 each. Rims and tires were $300 for five rims and tires, load rate E.
I mounted the axles by myself and welded them to the frame and because I was by myself that part took a good afternoon to finish. The side supports and stake pockets took about a day as well. If I recall, it was about 40 hrs of work and $1800 total cash out.
I think the biggest benefit on building your own is the fact that its to your own specifications, just like you mentioned.
I needed something to haul 30+ foot logs so I built a 30' flatbed trailer.
I will see if Buzz has the pictures he took from the build process as it was a fun adventer for both of us.