I've only recently started collecting, only about two years ago actually. I've gotten most of the comics (that are on my wall at least) from ebay.
The thing that started this all was actually my sister suggesting I decorate my room with something. I thought for a while and decided comic books would be something cool to do that with. I had always liked batman as a kid, but I wasn't really into comics. So for my birthday my sophomore year in high school (I'm just about to start my freshman year in college) my dad bought me a set of 150 comics for about $50 off of ebay. Ever since than I've been buying more and more stuff on ebay (Full LODK set, full Gotham Knights set, etc) as well as buying stuff I see at stores.
Its a lot of fun to collect something you know you'll never really complete. There is just so much stuff to get and you always find something new. I really enjoy it.
I also have no idea how much I've spent. I would guess everything I own total would probably be worth ~$2,000, but I'm really not sure. If I sat and thought about it I could probably come up with a better estimate, but I rather not as I would probably be shocked at how much I spent, lol.