They are all simple F-N-R double-reduction gearboxes.
The FL400R has the internal clutch.
Both FL350R and FL400R can only run their respective axles.
There has been success with running the stock Honda gearboxes with the 440-to-670cc Rotax, and there have been some stock gearbox failures with the stock
Engine (both extremes).
RPM has just been built a little more heavy duty, horsepower "rated" & proven, is available in common CV output flange-types, CVT or sprocket input, and available in a few different ratios. You pay for the availability, flexability and the investment someone has undergone to make it all happen. The RPM is also not made at the "Honda production rates/volumes" the stock Honda gearboxes were in their hayday. Economy of scale dictates the "more specialty-type / lower volume parts from a smaller outfit with less-than-Honda cost leverage on everything" will demand a higher price.