Price is variable depending upon where you are selling and who is buying and how bad the buyer really wants your particular rifle. There's a phrase for this.
Your rifle looks solid, but not terribly unusual. But it's not butchered by Bubba and could, maybe, get between 3 or 5 bills. Maybe more if you run into someone who really, really needs only a Nagoya 4 to complete their collection. I think that you should buy a good reference book. If you bought several, then you might as well be an informed buyer and seller going forward. Additionally, you could spend a couple months watching auction sights for Nagoya Type 99 rifles in condition similar to your rifle.
Nice rifle, btw.