It is very easy to buy half a deck online for cheap.
2 Juniper
4 Professor Oak's New Theory
4 Dual Ball
4 Pokemon Communication
4 Junk Arm
4 Stantler/Manaphy
Those 26 cards will make any new deck viable. All the cards there put together will likely cost less than $10. The additions to that need to be a) Energy, b) Rare Candy, c) Pokemon Collectors and d) whatever else.
I recently started playing this year. I started the game by simply buying $70-80 dollars worth of staples to form the whole deck. What I tell to you (from experience) is, unless you plan to jump straight into competitive, don't worry about a perfect deck... If you want a stage 2 deck, you will need rare candies, for instance, but you don't NEED rare candies to start playing.
I don't want to be modded for advertising, but I suggest trollandtoad. They made it easy, with $1 shipping (i live in australia) but you can find other places as well. If you attend a local league, it is likely people will trade you stuff.
Edit: I would suggest HEAVILY AGAINST buying one box of each set.
My suggestions are only to buy a) Unleashed Box(es), b) Triumphant Boxes, or c) HGSS starter kit.
HGSS starter kit comes with 2 pokemon collector and 2 pokemon communication (and a booster pack).
Unleashed boxes have tyranitar prime and kingdra prime, among others; as well as rare candy.
Triumphant has all most of the good primes.
HGSS booster box... the only good pull is Collector or Donphan or DCE (which is now a league promo) (sans jumpluff) and you're likely not going to get much.
UD booster box... all the primes are bad (possibly except scizor) and RDL will hardly get you anywhere.
CoL Booster Box... there are no real good cards in this set for starting the game. Pachirisu is trade bait, but the set overall is TERRIBLE.
BW Booster Box... You might get ability emboar or zoroark, but that's about it. Emboar's coming out in a tin soon, and zoroark is already a promo.
My money would be on Triumphant - simply put; if you get a magnezone or a yanmega, people will trade you almost every card you need for them.