This varies greatly gun to gun. Figure without any further info or the ability to properly inspect the gun you should be able to land 350 bucks to 550 or so for a shooter grade gun that's not pristine depending on where you live. I would start your ad at $500 OBO and take what you get. On the other side, where are you located? I might just have to take it off your hands. The scope does not detract from the value. It would only be important that a gun had or had not had a scope mounting modification if it were collectible but the 99 you describe is not. It's a working gun and of serious interest to a lot of people that love them but it's otherwise nothing special that I can see. The most valuable part about it is that it was made in the golden age of guns when things like impressed checkering and MIM and plastic weren't common and the guns were thence much more durable.

Pristine examples, especially take-down guns and those in odd calibres can command much higher prices. Shooters, not so much.