The ideal way, at least in my opinion, is to buy a box for $80 and then ebay/trade some of the less useful cards to finish off what you need for a deck. You probably won't pull a full strong deck, but you may get 2 of the Stage 2, for example. In my EX1 box, I pulled 2 holo Blaziken. In my EX5 box, I pulled 2 Exploud. So, you won't always get the best cards, but you'll have a strong base, and won't have to worry about finding specific cards when you need a new deck.
I don't really get to do my ideal way though. I end up just getting a box and that's it, as there is no League here, and I never got into large-scale internet trading. Out of a box, there are 36 reverse holos. People will take reverse holos in trades, and many times, they'll be worth a nice amount to you.
Also, with my luck, I wouldn't ever just buy the singles for a deck and that's it. When I would try something like that, it would turn out that the deck wasn't great, and I'd have to change it. At least when you get a box, you have options. Just think of a box as a number of halfway-completed good decks.
I've been getting cards mainly by the pack/box since 2000 already. And I do find ways to put $80 aside every few months to get a box of every set. Not the easiest thing to get that much money together in a relatively short time, but it's worth it.