Conndition and originality are everything in a collectable old revolver,
just saying a model 15 is really not enough info to gauge value.
If its a clean shooter roughly $300 + or - but a NIB 15-1 or pre 15 might bring 3x that.

Briefly the 15-1 is probably the rarest but the 15-2 and older versions with original diamond Magnas bring a premium especially the 4 and 5 screw varients,

Dash 3 and 4 were the last pinned barrel versions,

Late dash 4's stopped getting the barrel pin and were the last of the tapered barrel 15's.

The model 15-5 and 15-6 had a heavy barrel like the 4" 14-3.