FRIED1765: I highly value your posts, less this one.
For a <2,000 pound tractor, a $3,000 trailer may suffice but the OP is now considering heavier.
I have owned trailers but never a trailer suitable for a tractor. Reading T-B-N, it seems selecting a trailer suitable for a compact tractor is as complex as selecting a tractor.
What about trailer brake controller on the truck? What about truck fuel and truck depreciation? What about trailer and trailer-load supplementary insurance? What about trailer tags? What about opportunity cost of OP's time?
If a dealer will transport for $2.00 per mile, only trying to cover cost of HD truck, HD trailer, ample insurance and full cost of driver/mechanics time, I doubt a tractor owner with a suitable truck and suitable trailer and liability insurance can SAFELY haul for less than $2.00 per mile, valuing tractor owner's self-transport time at "0". Each round trip (180 miles) will consume most of a day, with loading, tying down, driving 180 miles, unloading, waiting for service, communicating tractor problem(s), fueling truck, a meal, etc.
A dealer has to size truck, size trailer and size insurance for the heaviest, most expensive tractor dealer is liable to haul during the year. An owner with lesser needs will not, but all the same expense categories are there.
I have owned three tractors. I am pretty good at numbers. I have never been able to figure how I could trailer a tractor as cheap as my dealer, who has the advantage of spreading depreciation, tags and insurance costs over 300 excursions per year.
I am six miles from my tractor dealer. I am charged $50.00 for each transport.
6 miles X 4 segments = 24 miles $50.00/24 miles = $2.08 per mile.
THE DIVERSITY OF OPINIONS POSTED ON T-B-N IS ITS STRENGTH.
New trailer would certainly have resale value in excess of $2,280 ($3,000- $720) after only one trip.
Trailer will always be available for future needs.
You cannot sell trailer after one trip AND have trailer available for future needs.