Long story short, there are cheap decks, but if you are just starting, I cannot think of any way to make a deck under $50. There are staple cards that need to be run in every deck, with a total cost of around $35. If you want to save some money getting these staples and being able to build a good deck, I recommend you buy 2 Red Genesect Boxes (around $20 each). They come with 2 copies of the most common staple cards like N, Juniper, Ultra Ball, etc, so buying 2 of these will give you 4 of each. With your last $10 maybe buy 2-3 booster packs from Legendary Treasures to potentially pull an EX. With all those cards you can amass a decent deck. But I'm afraid without common staple cards, there aren't really any decks under $50. If you happen to have most of the staple cards already though, do what GadgetJax said ^^^^. Flareon is an incredible deck and very affordable. It was my first competitive deck, so it has a special place in my heart <3