Hard choice between video games and horror memorabilia as my major collections. Neither are cheap, of course. My cheapest collecting addiction, however, is bottle caps. Video games are pretty much anything under the sun if I can find it for a good deal. Horror memorabilia is pop horror, like slashers, Army of Darkness, or classic monsters.
Video Games: Collecting for three years now and I kind of adopted the love for collecting due to nostalgia and the fond memories I've had connected to the Nintendo 64. Just kind of lifted off from there.. trying to reclaim one's childhood isn't cheap, I will say.
Horror Stuff: Just over a year. Adopted this one on my own because, well, I love horror anything.
Bottle caps: Nine years (weird habit I picked up as a kid). Every time I watched my dad drink, he would always pop the caps off in a weird way every time and the different shapes that came about always made me gitty and fascinated. Then foreign versions of popular sodas or beers came about when my neighbor came back from a month long trip to Mexico and Brazil he gave me the caps (he also collected bottles, one of every drink he ever had alcoholic or not). The interest just kind of came to me and I've always been made fun of for it!