It's very rare for the EV of a box to exceed the purchase price of the box (plus shipping, etc.) and then to capitalize on that, you actually have to go to the trouble of selling them and trying to get book value on the less desirable cards, etc. If you factor in your time on top of that, you're still probably making less than minimum wage, even if your box EV is way over the purchase price!
I buy boxes because I like to get home from work, grab a tasty beverage, pop my feet up on a chair and crack a few packs and read every single card and stew on brews for a while. It's entertainment value for me. As burn your face mentioned, you're much better off buying the specific cards you need for a deck as more then likely, if you want to build a competitive deck, you'll only get a handful of the cards you need (since you'll need multiples of rares and uncommons) out of cracking a box. That being said, I only had to buy about 6 cards to put together the new "12 bolt" modern deck as I already had most of the cards from opening boxes over the past couple of years and it uses a bunch of high volume cards. (i.e. I had about a dozen fiery tempers, several bomat couriers and soul-scar mages, etc.) :p
If you're looking for cards to play, I'd buy those individually. If you're trying to be cashflow-positive on it, you probably should look at other sets. (several sites publish EVs for boxes, so if you can get them cheaper than that, there you go.) I've had really good luck with Oath of the Gatewatch fat packs because they have a set of the full art lands that go for more than half of the purchase price of the box by themselves (and I like to put them in my decks... fire-suit on!) and you have a chance at "lottery cards" in the expedition lands and there are lots of creatures that are worth a fair bit (
thought-knot seer, kalitas, tons of big Eldrazi) etc. Plus you get
reckless bushwhacker and
wandering fumarole!