Undervalued? Pump Shotguns like Rem 870 Moss 500 probably the most versatile gun on the planet.
I would argue that pump shotguns are overvalued. When I first got into shooting, everyone was raging how you have to have a pump shotgun for home defense because it is THE home defense weapon. Now the debate rages between semi-auto shotguns, carbines, and handguns.
I think there are a lot of things that are overvalued not in price, but in what people think everyone needs to have. So even though they're cheap, I argue .22s are overvalued, because I do not have one, will not have one, and resent everyone saying "it's not a real collection if you don't have a .22 rifle and pistol." I would also argue that anything that becomes "everyone should have it" is something that at that point is overvalued. I do not need a bolt-action rifle, a revolver, or a 1911 for my purposes, so I'm not sure why many people think I need one.
As far as undervalued goes, I would have to say that anything that is not "standard" seems to be very undervalued as far as manufacturers are concerned (not worth their time to build a gun in that caliber) but overvalued price-wise. I'd be interested to see more manufacturers build 5.7x28mm pistols, .50 GI pistols, or even see someone come out with a cartridge on the lower end than 9mm, something like a 7.75x23mm straight-walled cartridge. But alas, such things aren't going to be as big of sellers (at least not initially) so they aren't going to do it.