card-making is one of those crafts, like scrapbooking, where you can spend a LOT of money if you're not careful.
and the worst part is that if you just mix-and-match store bough supplies, your cards will end up looking like everybody else's! in which case, what is the point of making them yourself? (this is a pet peeve of mine! the homogenization/corporatization of crafting!! but i digress.)
i would say, set yourself a budget, and try to substitute creativity instead of purchasing stuff. challenge yourself to make the coolest cards you can with what you already have on hand.
like, instead of purchasing patterned papers, why not try making patterns of your own using your computer and printer, or using pencils / paints / pastels you might already have.
instead of using (ridiculously expensive) self-adhesive dots or double-sided tape, use regular glue applied with a small sponge or brush.
use wrapping paper remnants, graph paper you already have, pretty pages from magazines and catalogs ... check the thrift stores for old wallpaper, children's books, old encyclopedias ... cannibalize old cards for images ... use fabric remnants, ribbons, lace, string, yarn ... you probably already have tons of stuff around your house that would look great on a card.
when you DO buy things, spend your money on supplies that can be used for multiple projects. for instance, buy glitter and glue instead of special adhesive glittery shapes. (the one time adhesive stuff is where they really get you!)
