I used to play MtG in early 90's with some of my friends that were besotted with it (and many of them still are). I never bought my own cards because as a poor student I couldn't afford them. Well, in fact I bought one The Dark expansion booster pack – it's still unopened. Even I didn't have cards my friends had them a lot so it was not a problem to make decks and play.
A decade later my son started the school and he became affected to many many thing we've here in the western civilization. One of them was Pokémon trading card game. The cards were expensive and for me the game looked boring, thus I ended up to buy him a MtG instead. You get the cards cheap e.g. Safe Haven (located in Helsinki, Finland) sells one kilo of common MtG by 10€ (price erosion has been notable as in 2004 the price was 13€) and one kilo of uncommon MtG by 35€. According to Safe Haven one kilo equals approximately 600 cards making 1,7snt/card for commons and 5,8snt/card for uncommons.
The cards are also sold in several on-line marketplaces. I have bought many cards from Merzenarie. He's also present in popular Finnish on-line marketplace huuto.net.
My Champions of Kamigawa collection.
Check Wizards of the Cost's Magic: The Gathering web site. Everything is there.