Since model 99's haven't been made since the 80's they're becoming hard to find, especially good unaltered ones.
The worth of your 99F depends on many factors including what the criteria of "very good condition" is. Guns are graded by percentage of overall factory original condition which includes what the condition of the wood, metal, blueing, sights, screws, butplate, case color hardening etc. is.
ANY alterations to the origional factory condition or model specifications such as adding sling swivels, drilling & tapping the receiver, changing butplates or adding recoil pads, changing sights, reblueing, redoing stock finishes etc. greatly detract from the value of the gun.
Also the model 99F was made as a pre-war takedown model from 1920-1942 and a post war solid frame model from 1955-1973. Value is almost twice as much for the pre-war model 99F takedown.
Adding more info or clos up picture will certainly help to value your 99.