I love hearing people talking about why and how they build their collections. It's certainly a valid topic in the King community.
Kevin's approach to collection-building sounds similar to mine (albeit better-funded -- that complete collection of Castle Rock makes me salivate). My collection began in more or less the same way: out of a simple desire to read everything King has written. From there, I developed a desire to have all the movies, all the comic books, all the soundtracks. I've got some movie posters, a decent number of audiobooks, a lot of books about King.
And so forth. Essentially, I just collect the things I enjoy having. Couldn't possibly care less about value, although I do have one signed King book (an anthology signed by all the authors).
Like Kevin, I'm a big Joe Hill fan; and an Owen King fan, and a budding Tabitha King fan, too! I started buying their books just because it seemed like I ought to; happily, I've enjoyed all of them that I've read so far.