Last you checked must have been a while ago, given that that calculator was last made 37 years ago. I’m not sure what the modern equivalent (scientific but not graphing) would be, given that TI so dominates the standalone calculator market now, but they push their graphing calculators so hard. A used HP-15 probably would still be better than whatever TI is making, but we’d be looking at long-past-used prices for both the calculator and the slide rule, and both probably vary wildly.
There probably are a lot of high-quality used slide rules that are owned by people who’d be willing to part with them cheaply, given that most people nowadays don’t even know how to use them. Heck, a couple of folks have given them to me for free. But by the same token, because they don’t know what they are, it probably wouldn’t occur to most people to offer them for sale. On the other hand, most people who still own an HP-15, even if they think it’s obsolete (it’s not), would at least know what they have.
Another confounding factor is that if a calculator was stored for decades with batteries in it, the batteries might have corroded enough to ruin the calculator. But there’s no equivalent failure mode for a slide rule.