The value of Japanese swords is largely affected by their blades. Particularly with "old swords" or "new swords" that were used by the Samurai, the sword smith who made and signed the blade, how the steel blade is made, adorned or worked, the condition of the polish, etc. all add or detract from the value. Japanese sword blades can be made of the highest grade of steel called "Tamahagane" or lower grades of steel often generically called "railway steel" or "machine made blades" or even stainless steel blades.
Sword blades made after circa 1877 cannot correctly be called Samurai swords since that warrior class was dissolved by the Japanese government after that date. These are in a different class from later Japanese military swords even if they have Tamahagane blades. Old & new sword blades from older katana and wakizashi could be mounted in later gunto so collectors will sometimes find old Samurai blades in rather common military or civilian mounts from WWII. There are several general classifications of Japanese swords made by collectors and one is delineated below by Fuller & Gregory in "Military Swords of Japan 1868-1945":
Ancient, pre-900 AD
Koto (old swords), 900-1596
Shinto (new swords), 1596-1800
Shin-shinto (new-new swords), 1800-68
Modern swords, 1868 to date
Having said all this, what is Tamahagane? Tamahagane is the highest grade steel made in Japan for sword blades. This steel is made in an open Tatara furnace that is heated, and tended, for three days and nights to produce Tamahagane. Iron sand and charcoal are continuously added to the furnace to make one ton of steel from 13 tons of iron sand. Only one man, Akira Kihara, still makes Tamahagane for sword blades in Oku-Izumo in the Shimane region of Japan. There is a legend that Susanoo, who was one of the gods who founded Japan, slayed the eight headed dragon Yamata no Orochi (known as the Great Serpent) in Izumo. He took a sword from the body of the serpent and it remains a part of the Imperial regalia to this day.
So, Tamahagane blades are the epitome of Japanese swords and command the highest prices all else being equal. A good example of the saying, "buy the sword and not the story" when it comes to collecting Japanese swords.