I bought and installed a NEW factory Kubota ROPS for my B7100D (gear drive). I believe that I paid just over $200 (after taxes) for the unit, and you should be able to come close to that price. I had looked at a used ROPS, and the guy was asking $600 (I guess before Kubota started selling them at cost they were a pretty expensive option).
The MODEL number for the 2-post ROPS was B1060. I'm not sure how the serial number you gave will help.
According to the ROPS manual, it fits "B5200E/D, B6100E/D/HSE/HSD, B6200E/D/HSE/HSD, B7100D/HSE/HSD and B7200E/D/HSE/HSD Kubota tractors without backhoe".
If the model number I gave can't help you, then the only other thing that I can offer is that the
B7100 ROPS (with integrated PTO shield) is mounted in place of the PTO shield that should be on your tractor. To get the existing PTO shield off, you'll have to remove the 3-point hitch rod that runs through the bottom of the PTO shield (which will require removing one of the rear wheels), and remove the 3 bolts on each side of the PTO shield. Installing the ROPS is easy if you have two people to hold the ROPS in place while a 3rd person bolts it in place.
Does the used ROPS you're looking at include the seat belts?
Hope this helps,
Kelvin