Yep, to me that's $100 gun max. That's about what they seem to go for around here and that I'd subtract value for it being modified, but maybe add some back for the scope. IMO that's a $20-25 scope at best. So I'd honestly say $100 is the max I'd give for that gun, to be truthful, I don't like modified items as you are often getting into someone else's headaches so I'd stay away from it all together.
As for your gun, it depends on which one it is. If it's the heavy barrel version with the accu trigger and just regular blued barrel and synthetic stock, I'd say it's worth at least $200-225 around here. If it's fancier it would bring more, and without those two features less, but I don't think it would be a $100 gun no matter what. So either way you look at it, I think you are trading for a gun that's worth a lot less than what you have. The only way I'd trade and this was if I had to have that gun would be for him to throw in at least $100 cash if not $150.