Depends how long you're coming back for, how big your collection is and how big your wallet is. About 150 packs from each Standard expansion should get you close to a full Standard set with dust left over to craft what you want. Then you'll have the momentum to freeroll for future expansions as long as you clear the dailies every day.
No you'll have the police at your door if you try getting older expansions. :P
But everything past Old Gods is usable in Standard, so that would include
1) Whispers of the Old Gods
2) One Night in Karazhan
3) Mean Streets of Gadgetzan
4) Journey into Un'goro
5) Knights of the Frozen Throne
With one more expansion yet to come before the first 2 cycle out.
You can still buy packs from older expansions if you want, but it has to be done through the Battle.net shop I believe now, and those are only usable in Wild.
But as Twoddle said, if you dump that much money into the game for 50 packs from Gadget/Un'goro/Knights each, yeah, you'll end up with more than enough to make a few decks probably, and enough dust to make probably roughly 3 legendaries in the end.