For casual play we allow proxies of pricey cards (typically those that cost $5 or more a card)... Proxies are really easy to make - print out a color copy of the card face, stick it in front of a land card in a sleeve.
We play casually here too, but I'm torn on the subject of proxies.
On the one hand, it's a cheap and easy way to play-test a deck before you shell out hard earned dollars. On the other, I'm starting to see a lot of decks that would cost > $200 to actually purchase...
I don't think there are any tournament-level decks that would cost less than that to purchase, are there? My elf deck has all real cards and it's worth slightly over $200 even though none of the individual cards is worth even $10. If you had one set of $20 or more cards in your deck because they are the spine of a combo, keeping the deck as a whole under $200 would be impossible. If you're playing multicolor, you really want at least that one set of original dual lands. And it's unusual to see a tournament legacy deck without a set of force of wills. If you're not building tournament level decks I don't see any point in ever replacing the expensive proxies with real cards.