Banebots will be the only one to answer that question. I can provide some related answers you might be interested in, however. Namely, what other options you have.
Andymark has the
GEM500 gearbox 13:1 2-stage, 49:1 3-stage. Or the
PG-71 gearbox at 71:1.
From Banebots, you could buy all the P60 parts individually and assemble the thing yourself. It's about $25 more expensive (Crazy, I know). I can post a list of all the necessary parts for a 20:1 or 26:1 gearbox if anyone's interested.
In my search for gearbox alternatives last year, I came across some possibilities that I've yet to try. WARNING: These look like they interface to an RS-500/550 based on the gearbox drawings. I can't guarantee that, nor do I know anything about the robustness or reliability. Most have the motor attached, so you'll need to press the pinion off the shaft or find a new pinion somewhere.
Robot Marketplace PK27 (27:1) (sounds spur gear, less robust)
Robot Marketplace PD27M (27:1) (planetary, more robust)
Also possibilities if you can find somewhere to buy them:
http://www.mfacomodrills.com/gearboxes/942d_series.html
Or any gearbox/motor off this page:
http://www.kysanelectronics.com/Prod...p?recordID=247