A used rifle is worth exactly what you can get for it. I tend to agree that the values of used, off the shelf 700s, even those built on short runs, don't change much regardless of the cartridge they are chambered for. After all, a unique chambering is only a reamer away. If I wanted a 700 in 250 or 300 Savage, I would locate a donor action and have a custom barrel spun onto it, in which case yes, it would be worth a few dollars more than a used factory 700 . . . to me at least, due to the quality of the barrel, not the chambering. With respect to a 700 in .300 Savage, it would be less appealing to many than similar rifle chambered in .308 or a .30/06, thus worth less to anyone other than a .300 Savage enthusiast or a collector of 700s.