What actually defines a casual deck? For one, a casual deck shouldn't be based on any form of tournament deck. Of course, it's not entirely impossible that a casual player would stumble upon a supremely powerful archetype during deckbuilding. Is a casual deck necessarily cheap? No. In fact, I might have probably built physically one of the most expensive casual decks to date. It's difficult to pinpoint what kind of deck is actually casual any more, but I believe that there are only few very clear rules: 1. Isn't based on a successful deck archetype. - To me, this means anything at the current time and before that. Actually, casual decks could still be based on deck archetypes, and still not be too competitive to be fun.
2. Wins in the most interesting or fun way possible. - Some casual decks have normal ways of winning, but certain combinations of cards (like in preconstructed decks) can lead to some really fun and close matches. Some casual decks just try a fun way of winning - Mycosynth Affinity, Spiritcraft, Panoptic Mirror, some obscure combo.... and so on. Competitive wins are generally unstoppable to casual decks, with some exceptions, of course.
3. Should have a net worth of below USD100 if possible. :D - Well, it has to be capped somehow. I don't think it's correct to call any deck running 4 Wrath of God, 4 Umezawa's Jitte and 4 Cranial Extraction anywhere close to being casual.
4. Can adapt easily to a multiplayer environment. - This is one of the few things people keep forgetting out of casual. Most casual decks are designed to handle both variations of player environments with ease. Hah. Some casual decks don't - making this rule a gray spot, but most casual decks should have no problem - meaning that the deck you play doesn't trade off resources too quickly for gains.
There are many forms of casual decks, and some of them even go up to become supreme rogue decks at tournaments after some tweaking. Of course, I believe that most casual decks are actually also peasant decks.