I think that's a tad high. I'd put it in the $2.5K range for 98%. 3K really only if it's unfired and in the original box.
This sounds more correct to me.
The B-Square mount isn't worth anything.
The Leupold scope might be what's pushing the value up to $3200, but 6.5-20x sounds like too much magnification for an HK91.
The problem is that scope mounting on an HK9X series inevitably puts scratches on the finish, just because of the way the mount has to clamp to the receiver. I would be skeptical that a 98% gun with scope mount on is actually at 98% gun once you remove the scope mount.
Idiosyncrecies on an HK 91 that are going to affect value:
Finish Color: Blue/Grey or Black. Black is more valuable and indicative of later production.
Chipping around the ejection port: HK's toss brass hard and this area takes a beating. It's not repairable, but can be covered up with a port-buffer.
Stock: A2 is a lot more comfortable, but some came with A3 stocks that are expensive, but pretty much a joke/torture device to shoot with. Chance you might find an MSG-90, PSG-1, or G3SG-1 stock (which are all $$$ upgrades).
Trigger pack: Most likely you have a standard trigger pack, but worth it to see if the previous owner might have done work to it, or better yet, upgraded it to a PSG-1 trigger pack, or the even more rare SG-1 trigger pack (crazy rare and expensive).
Trigger housing: should still be metal/plastic 0-1, but some people upgrade the all-plastic pictogram housings--check to make sure it's authentic HK if it's not the metal/plastic 0-1.
Personal preference: slim or wide forearm. Doesn't really affect functionality unless you have a preference between the heavy or light bipod (Heavy only works with slim forearm), but something to think about because if it's not what you want, you'd have to buy a new forearm.
Cocking Handle: When I owned an HK91, I replaced my HK91 small cocking handle with an HK21 cocking handle--much easier to grab a hold of. Not cheap though.
If there is a way that you can check the bolt gap distance, that would be a good thing to know.