MTG: SHOULD YOU BUY BOOSTER PACKS/BOXES OR SINGLES?

Tarmogoyf Future Sight
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I have been playing Magic: The Gathering since Onslaught back in 2003. Every since then, I have learned one important thing about investing in MTG and building a deck: It's better to spend your money on directly buying the cards you need for your deck rather than buying packs or boxes in the hopes of getting them.

You'll have friends and other people telling you to go for the box (not the packs to save a couple of dollars), but it's more practical to get the singles even if your deck would cost you about $200. If you want to continue being a Magic: the Gathering player (competitive, casual, online or not) in the long run but you're not a millionaire, you've gotta use your dollars wisely. Additionally, not all booster boxes are worthy investments. Sure, let's look at the most recent successful sets (in terms of their value):

ZENDIKAR

Zendikar Booster Box
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Let's all be clear about it. Back in my day, each booster pack of Onslaught would only go for about $2.25 each, going to about $2.50 or so when Mirrodin came out (oh, the dreaded days of broken affinity). I don't exactly know when the price increased again, but by the time I came back playing Ravnica: City of Guilds, packs were selling for about $3 each. Anyway, while it does look good to have invested in Zendikar booster boxes (especially the first ones to come out because of the "priceless treasures") as they were only at $90 or so each back then and now at about $500 each, sets like these are numbered.
WORLDWAKE

Worldwake Booster Box
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Take the next set in Zendikar. Indeed, it does have the most powerful planeswalker card yet in the form of Jace, the Mind Sculptor, but beyond that, the price of the booster box has only doubled after four years.
Recent sets such as Born of the Gods and Journey into Nyx are also not worth the investment, with prices of booster boxes falling to as low as $60. Investing on boosters are risky and rarely turn you a profit. If they ever do, it would probably take you a long time. Basically, if you don't have that much money to spend for MTG, go for the singles. Sure, you can buy the fat pack if you want the physical visual guide, the dice, the box, and that little poster included, but beyond those, they're still just boosters that could either have great, bad, or average cards. And you won't ever know because of the random distribution.

The best thing about going for the singles is that you are certain of what  you are getting. Sure, sometimes the card you just acquired might drop in value, but singles (or rather, four copies of the same card) will be easier to sell than a whole box.

Goblin Guide Zendikar
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Moreover, it rewards the player because you have to think hard about which cards should be acquired immediately and which should have their prices lowered down first. Personally speaking, it's better to invest in the Onslaught/KTK fetch lands rather than boxes of Onslaught and KTK, since those cards are not going to go dirt cheap, ever.

However, I also admit to buying packs and boxes, because the thrill of pulling a great card is so addictive, though of course it's more believable to have a $0.30 rare. This is also probably why I immediately acquire a pack in Hearthstone whenever I get enough gold. But seriously speaking, go for the singles first before splurging with packs, regardless if you're building a deck or you're investing in MTG.