Great score! I'm no expert, not by any stretch, but referencing the RMK 'Timeline' that my wife and I put together, I believe the following holds true
H.H. Heiser started making sheaths for RMK around 1943, and your RMK model #3 was introduced in 1943 as well. (The '6' on the reverse of the sheath is the blade length.)
Riveted sheath mouths were originally used on these knives from 1942 through roughly 1949, with the smaller rivet's like yours used in the early 40's.
The left-handed sheaths like yours were only (originally) used until around 1946 IIRC
The 'old brass' hilt, and medium thick spacers, especially the blue center one like yours has predates 1950 or so, as does the radius of the choil, yours being more elongated than those of the early 1950's
Your blade grind is a very, very early version, and with the black painted steel snaps (by United-Carr) only being used from 1942 thru 1946, I'd say that you can probably date your knife with some degree of certainty as to having been made between 1943 and 1948.
The sharpening stone if present could help narrow this down a bit
but I'm thinking these dates are pretty close. The thing of it is, those years in the late 40's were really slow times for RMK, with old blade blanks being dusted off as orders trickled in, and sheaths could languish in the sheath bins for years at times
As to prices, we don't 'do' that part
it's what the market will bear on any given day. Nonetheless, good score!
Hope that helps you out a bit. For more info please reference our RMK Timeline as linked through my Sig line below
Mel
p.s. sams, good to see ya posting here again!