Normally, if you have a particular deck in mind, you'd buy singles instead of boosters. Boosters can supplement a card base or serve as the foundation for a deck, but you're almost certainly going to have to buy singles to do what you're going to want to do.
If a single gets high level play in the constructed formats, it tends to affect the price. Most cards are a pittance (Less than a dollar), but certain lynchpin cards can really test your budget. I'm building a deck that is ~$1100, but the majority of the value is in
Cavern of Souls (4 @ $72.00),
Mutavault (4 @ $12.99), and
Aether Vial (4 @ $39.99). Granted, this is for an extremely specific deck archetype that's tournament legal. It's going to be overkill for any casual games I play. There are budget decks (Less than $100) that would suit most people perfectly fine.
I play legacy with some of my older decks, but I don't expect to win anything. It's a fun format if you don't mind losing or if you're rich. Modern is good enough for any kind of competitive play to me. You can be viable with less substantial investments.
