I used to play pretty seriously, but lately I don't buy cards anymore. The last time I seriously bought new cards was the Kamigawa block :-P
My wife and I just make decks out of our collection, and play against each other. It's fun, and doesn't cost anything; if we really want to mess around with other cards, we just use Cockatrice
(My favorite deck back when I was still playing came mostly from Onslaught block, and was a mono-black zombie deck. Not the most imaginative thing ever, but I loved having a creature focused deck that usually won by means other than direct damage from attacks.)