I think that value is going to depend on model specifics. The 929 and 999 came in multiple versions and barrel lengths. More info is needed for an accurate assessment as Gunbroker does seem to have some vast price differences.
MODEL 929 SIDEKICK
.22 LR cal., 9 shot, 2 1/2, 4, or 6 in. barrels, swing out cylinder, plastic grips, blue finish, DA, round or square solid frame, DA, round or square butt. Mfg. 1956-1973.
MODEL 929 SIDEKICK SECOND MODEL
.22 LR cal., 9 shot, 2 1/2, 4, or 6 in. barrels, swing out cylinder, plastic grips, blue finish, DA, round or square butt, transfer bar ignition. Mfg. 1974-1986.
MODEL 999 SPORTSMAN SECOND MODEL (NEW FRAME, TOP BREAK)
.22 LR cal., 9 shot, 6 in. barrel, adj. sights, top break, DA, blue finish. Mfg. 1953-1972.
MODEL 999 SILVER SPORTSMAN TOP BREAK
.22 LR cal., 9 shot, 6 in. barrel, top break, DA, chrome finish, serial number letter codes Z, AA, AB, AC only. Mfg. 1963-1966.
MODEL 999 SPORTSMAN THIRD MODEL TOP BREAK
.22 LR cal., 9 shot, 4 (new 1979) or 6 in. barrel, top break, DA, blue finish, adj. sights. Mfg. 1973-1986.
MODEL 999 SPORTSMAN 1 of 999 TOP BREAK
.22 LR cal., top break, DA, 9 shot, 6 in. barrel, adj. sights, engraved, fitted wood case. Mfg. 1979-1986.