The riser was likely something like a couple of pieces of 1" square steel tubing out of the welder's scrap box welded between the trailer frame and the axle
bracket. Cost would be in the welding labor and would be what your local shop would estimate.
When people want to raise the whole trailer, one way to do it is weld pieces of the box steel that the frame is made from between the frame and bracket.
If you are considering having the welding done, also consider getting the weld-on bolt-on axle
bracket from Dexter that's welded to the frame so the axle
can be bolted to it. This allows later replacement of the axle as just bolt-on job with no welding required.
All of the above presumes you have something like a Dexter torflex axle welded to your trailer's frame.