You are half right, for perhaps the wrong reasons.

They did only make the 915 for a few years but the sheer number they produced in that time is RIDONKULOUS. There does exists a gujillion S&W 915 pistols.

The 915 does not get it's "value" due to scarcity, they 915 is the antithesis of scarce. You can't swing a sleeping feline without whacking at least three 915's. But the 915 is a fantastic pistol and has earned it's place.

There are much fewer 910's on the market for sure, no question. Compared to a 915, it is certainly "scarce" but that cannot bring it's intrinsic value in line with a 915 or the 5904 from which both pistols evolved. The 910 is a 915 with many more corners cut and cheapened. It carries inferior parts, including some extremely important parts, and it has the special bonus of being an UGLY version of a handsome pistol.

The true Value Line pistols (910, 410, 908, 909, 457) all have blocky slides that are missing the telltale angles and attractive look of the rest of the 3rd Gen pistols. And they each have plastic bits in places where 3rd Gens use metal parts.

Simple process of elimination says that in any amassed group... some will be near the top and some simply must be near the bottom. And the 910 pistol is the king of the bottom end of all 3rd Gens.