I like to think of cards as budget, not decks. I don't make my decks to be specifically budget, because when I have a
Food Chain or a
Sliver Queen or a
Gaea's Cradle just lying around, I might as well put them to good use. That being said, I'll rarely spend more than a dollar to purchase a single. If I find a card that's functionally very unique and necessary for my strategy, I'll go up to two dollars sometimes, but that's about it. Because of that, I spend between $20 and $50 buying cards for most EDH decks I build, but I also only buy about a third of the cards I end up using. I consider that to be a budget approach, buying cheaper cards that fill roles, only splurging for the expensive cards if they're really necessary for the deck, and never splurging on anything that's definitely expensive. This approach will allow you to play casual EDH, fairly competitive pauper, and, of course, regular kitchen table Magic. It will not allow you to be competitive in EDH, Legacy, or Modern (and usually not standard).
For those formats, the definition of "budget" can't be based on getting the cheapest cards that reasonably fill certain roles, because in those formats there are not cheap alternatives to necessary cards. The ability to win depends on having top tier cards, and those cards will never be budget. The design of gameplay at those higher levels of competition is not using cost or availability of cards as a factor in deck design, so decks typically cost hundreds, and even thousands of dollars, not because people are just filthy rich and want to spend that sort of money on cards, but because that's what it takes to be competitive. The mindset is, buy the cards you need to optimize your deck. In top tier, highly competitive formats, there isn't an idea of "budget" because that idea conflicts directly with the optimized deck. True, some decks are cheaper than others, but what I'm trying to say is, if all the reprints of
Lightning Bolt were suddenly destroyed, you wouldn't see a bunch of legacy burn decks running
Shock instead, you'd see the price of playing Legacy burn skyrocket. Burn's not budget, the best cards for burn just happen to be on the slightly cheaper side.