In modern guns??? Nothing. I can't think of a single current production handgun that I would consider "heirloom" material. To me, heirlooms are from the past. They're a physical connections to loved ones long passed, ancestors only vaguely remembered or never known. You can't buy that at a gun store.
That said, every family heirloom I have started out on a gun store shelf where someone in my family, long before ai was born, bought it. So with that in mind, I'd say my Ruger Redhawk 44 magnum would meet your criteria, but only because I enjoy shooting it and I carry it a lot. (It doesn't fit your price criteria.) And even at that, I consider it nothing more than a gun I bought. My son may see it differently in 50 years, but I doubt it as he will likely sell it the day after I'm dead.