"Die-cast" is shorthand for die-cast metal. If a toy has a substantial degree of metal in it, it's usually referred to as a die-cast. Dinky toys, Hot Wheels cars, Star Wars Titanium toys, etc., you could call all of these die-casts. They all have some degree of plastic parts in them.
The word you hear the most when referring to Japanese die-cast toys is "gokin," which is a shortening of "Chogokin." The word literally means "super alloy," and it's the word Go Nagai coined to describe what Mazinger-Z was made out of. In the 70s, Popy adopted Chogokin as a brand name for their die-cast toys, and today other companies use the "gokin" part of the word in their brand names (Aoshima's New Gokin, CM's Corporation's Brave Gokin, etc.).
A lot of people feel that the 70s and 80s toys, which were made in Japan, have a much higher degree of consistent quality than modern toys (which are now made in China, with a couple of rare exceptions).
Prices for new die-cast toys are relatively high compared to new vinyl toys due to their complexity (most of a toy's cost is based in labor), but compared to aftermarket prices of some Secret Base and Real Head toys you can get a lot of bang for your buck.
Vintage Die-casts that command high prices usually do so due to rarity, either because the company didn't make many, or sometimes they have fatal flaws which make them easily breakable and hard to locate pristine specimens of. Packaging is key for die-casts: most collectors expect to have the box and all parts for them, preferably on the original sprues.
Character is key, too. If a character was popular in Japan and the US, then twice as many people will be looking for toys of it. Voltes V is hugely popular in the Philippines among Gen X'ers due to the show being banned by Marcos in the 80s.
A good place to find out more are the books Super #1 Robot and Inside the Robot Kingdom, and of course the TBDX Data Files:
http://www.toyboxdx.com/datafiles/