The 2 cheapest decks which are available for tournaments are:
Quad Sigilyph or Speed Darkrai.
Quad Sigilyph is 4 Sigilpyh and 2 Mewtwo EX with alot of trainers and supporters, here is a good idea of a "skeleton" list here
Speed Darkai uses 4 Darkrai EX and 3 Sableye with a fair amount of trainers and supporters.
This deck can also include Terrakion which can help against enemy Darkrai EX. But needs the inclusion of fighting energy. This website shows a variety of different Darkrai decks, so chose the most basic one to start with, which is just pure Darkrai EX which is known as "Hammertime" here
Darkrai and Sableye are just a too powerful in this format to ignore and they fit with the darkness theme.
Mewtwo EX and Darkrai EX can both be acquired as single promo cards or bought as a collectors tin and the rest of the singles vary in price.
Most decks will use around 8-14 basic energy and some will use special energy such as Double Colorless or Blend energy.
Any good Pokemon deck will have a good supporter line up, so if you are on a budget, best to invest in those first.
I don't wish to be rude, but any hobby/game you have to invest money into in order to win tournaments.
Keldeo Blastoise would be a very expensive deck, which is why I avoided it, but it is considered "THE" deck of the current format.
A new set is being released soon and the meta-game will change slightly and may make Quad Sigilyph less viable to play.
I would advise investing more money into Darkrai EX as it's still a strong contender for winning tournaments and Sableye is considered one of the best "support" Pokemon of the format.