Most of the estimates from those who responded to your question are
on the high end. I checked my LGS's pricing resources and came up with
this: $650 as the starting point.
Models such as the 40XB Tactical can bring $2400 if they are in
original condition, have all the paper (manuals, proof of ownership,
etc.) and the original box would help.
A good online resource is Firearms Price Guide.com, you can look it up
yourself.
http://www.firearmspriceguide.com/guns/search/remington/100
ALL of the shops I trade with have the same policy regarding so called
"custom" or after sale work/accessories/parts and that is they are
worth nothing, they only go by book value and condition. Many shops are
becoming reluctant to buy any gun that is not in original condition due to
liability. So unless they did the extractor, some shops will low-ball you.
Now if you can find a private buyer who likes what you are selling then that's
the best route to go.
Ted