To just give you an idea of the cost of the investment into the staples that go into just about every deck, ill make a list for you. Ill edit it in in a few minutes.
4 Professor Sycamore @ $.75 ea, $3
4 Trainer's Mail @ 1.25 ea, $5
4 VS Seeker @ ~$10 ea, $40
4 Battle Compressor @ $4.50 ea, $18
4 Professor Birch's Observations @ $1.75 ea, $7
4 Acro Bike @ $1.50 ea, $6
4 Ultra Ball @ $.25 ea, $1
4 Level Ball @ $.50 ea, $2
2 Shaymin EX @ $35 ea, $70(Optional, But highly recommend for Consistency)
Grand total ~$80 investment into STAPLES used in just about EVERY deck.
These are the most commonly used cards in deck creation, in my opinion. Most of the time they are ran as a Playset, or close to it, besides Birch. So you can see, the investment into just the staples can be little overwhelming at first, but once you have these cards, you could build a large variety of decks. The only other additional investment will be a couple One-ofs of cheaper cards, and your pokemon lines, and depending on the deck you can get all the pokemon for a deck for surprisingly cheap, cheapest being probably Vespiqueen / Vileplume, or even Night March(Except for Shaymin EX), both of which are highly competitive decks in the standard meta at the moment.