As with any firearm, it is not possible to accurately appraise a gun without seeing it. Recommend that you see a local gun dealer for an appraisal. You can also do your own investigation by looking at other models being sold that are in the same condition as yours. Try the gunbroker.com or a periodical called "The Gun List" You can also reference the Blue Book of Gun Values which can be purchased on line at various retailers.
I don't think there is an invector plus model. Invector Plus is the choke system Browning uses on newer shotguns.
Browning did not make an Invector model. That is the type choke system of the shotgun.
The 2 letters NX in the middle of your Browning Invector pump action shotgun indicates that it was made in the year 1993.
50-550 usd
Its about $550 @ basspro
What gauge and grade is it?
A 12 gauge shotgun with changeable choke tubes that fires 3.5" shells.
Try the links below
B-80
300 USD or so
The Invector, or "Standard Invector" system" was introduced in 1983. A few years later, the Invector-Plus chokes were introduced to fit the new back-bored shotgun barrels. These two choke systems will not interchange. If you have screw-in chokes in your shotgun barrel, the kind of system you have will be stamped on the barrel near the receiver. You must use the proper choke system for your shotgun.
Browning Choke Markings "* " = full "*- " = improved modified "** " = modified "**-" = improved cylinder "**$" = skeet "***" = cylinder