reb87, S Grades are in the time range of the R Grade. Daryl could go much further with this than I can hope to but, I will try to fill in as much as I can. S grades could be considered to have replaced the B Grade.

R is roughly the equel or replacement of the A.
S "" '' ''' of the B.

Is $3000 too high? For a re done gun? I think so, on the other hand keep a record of how long until you see the next one for sale.