Instead of the usual range of values, SCSW simply says "Too rare to estimate value. Most specimens would probably sell in the low four figure range." Now, that's for an actual Model 20, which would be post-1957 when model numbering started.
However, the serial number you listed isn't correct for a Model 20, which would have an "S" prefix followed by 6 digits. It would be in the right range for the .38/44 Heavy Duty (Prewar) (or the Outdoorsman, which is the same gun with adjustable sights), either of which could be thought of as a pre Model 20, although that designation actually is used for the .38/44 HD Model of 1950, which would also have an "S" prefix followed by 5 or 6 digits.
Assuming the SN is correct and the gun actually is an original .38/44 Heavy Duty or Outdoorsman, 60% finish on a gun of that age would place it in the "Good" to "Very Good" range, assuming that it is in good working order. As such, if the gun has fixed sights it's probably worth at least a couple hundred dollars more than the price quoted, and if it's the adjustable sights Outdoorsman it's worth a few hundred more than that.
Edit: I see carguychris beat me to it, with substantially the same info. As he said, run do not walk back to the store and buy the gun. That number doesn't sound like the serial number, which would be found on the butt of the grip frame.