In summary;
I bought a complete power steering system off a John Deere 400, which included 4' cylinder, control valve with drag link, hoses, and a hydraulic block or manifold for $250. On the JD400 the steering radius is about 6" and JD designed the cylinder to be placed/drilled hole in the steering arm about half way. I didn't think the steering arm on the B7100 was heavy enough to drill a hole in it, so we tried to design a bracket to attach to the arm. It just didn't come together, so I bought a 8" cylinder ($45) and assemble it as MrPaul did. His pics of the control valve/drag link and the cylinder are very similar to mine.
We taped into the existing hydraulic system by adding a divider ($86.95) which provides 1.5GPM continuos flow to the control valve (power steering), and all new hoses ($200). My tractor has only 450 hours and looks in new condition so I wanted the power steering unit to look as good, otherwise I could have cobbled in the JD hoses and save the $200.
So my total cost for parts was less than $600 and its worth a lot more considering the convenience...
If I was considering adding power steering and didn't have the resources for the project, I would buy the hydrostatic kit (system) for $1,000 and hire the installation. The hydrostatic system is simpler to install, plus you would have the same system the new Kubota tractors come with. I would guess the total cost for a hydrostatic system including labor and hoses may approach $1,500 but its still worth it considering resale value.
Clark