I saw a list from Metro trailers in Winnipeg from May 2017 that had the following prices for galvanized frames:
Frame only (no axle) $1600
Frame w/ idler
axle $1850
Frame w/ electric brake
axle $2260
So that should give you an idea of the cost from a shop.
They estimated 8 - 16 hours of labor to install the new frame on top of the costs above.