Saw one similar to this about two years ago in a local shop. Asking price was $100, but I could've bought it for $85 as the gun had been there for quite a while.
Since sidelock guns are not as popular anymore with all the "interest" in inlines for hunting; you could look at it logically with two opposing views:
1. No one wants this kind of gun, so therefore they should be worth much less than normal.
2. Since they are getting less common, they should be worth more than what they had been.
I have to go with #1 in this instance, 'cause any parts breakage or replacements needed, and you're probably gonna have a hard time. Also, I know where I could buy an almost new (slightly used) CVA Bobcat for $75, and the parts for that should be easier to locate for a few more years. Besides that you can buy brand new inlines for around $100.
I wouldn't go above $100 for ANY reason on that gun; but don't sweat it if you did, just shoot the thing to pieces and enjoy it!
Shoot Safely!
WV_Hillbilly