Gun Trader's Guide, 32nd ed, pg 496, Model 90 Standard Shotgun- 2 triggers NIB $678, Excellent $502, Good $400. Single trigger, add $66. 20 g, add 15%. Made 1937-1958. If the gun has been refinished, collector's value is cut a LOT. However, this is not a Parker or Merkel, and most of the value lies in being a shooter.
Condition and originality is everything. If it looks good, and you want a 20g O/U, you're in the ballpark.