decks are usually 100-300 USD right now the decks are more expensive since dedenne gx is around 35 dollars. if you dont want to spent too much starting out competitively I would recommend blacephalon/pidgeotto (100$). this deck is pretty easy to play, you use professor elm's lecture to search out your pidgeys and pidgeottos(team up) and use welder(unbroken bonds) to attach an energy to your(unbroken bonds) blacephalon. with this deck you need to manage your energy carefully and use Victini prism star at the perfect time so you can have enough energy for the rest of the game. this deck uses a one prize attacker so against 3 prize tag team pokemon you can play the prize trade to your favor. with this deck you don't need to attack every turn, only attack when you need to because your attack discards energy and most decks are playing mallow and Lana which heals 100 damage, so you want to go for 1 hit knockouts.
if you're ok spending more I would recommend Reshiram and charizard gx/abilities(210$) and Arceus Dialga Palkia GX(210$)
reshiram and charizard's plan is to use welder to attach energy to Reshiram and Charizard GX to hit for 230 damage, and the gx attack can do 300 damage and goes through effects like keldeo GX's ability. this deck just won the international in Sao Paulo so it is good right now, this is the list:
https://limitlesstcg.com/decks/?list=3122 ninetales is a gusting effect like guzma, lysandre, pokemon catcher, and gust of wind just to name a few cards you might know and you want to set one up to take knockouts on what you want to. this deck also has other attackers and Volcorana GX is also a great option to play to take 1 hit kos on tag team pokemon.
I don't have experience playing adp but you can watch youtube videos and twitch streams of people playing this deck.
stay away from control decks and mewtwo and mew tag team gx toolbox because they require a highly skilled player to do well.
This is a list of staple cards in the current standard format including card counts
4x Cynthia
4xLillie
4xWelder?
4xGreen's Exploration?
4xProfessor Elm's lecture?
4xProfessor Oak's research?
2xMallow and Lana Tag Team
2xCynthia and Catlin Tag team
2x Guzma and Hala Tag Team
4xTag call
You probably already have more than a playset of Switch
2xreset stamp
2xGreat Catcher
4xPokemon Communication
4xMysterious treasure?
4x Electromagnetic Radar?
4x Jirachi(Team Up)
4x escape board
2x pal pad
4x Pokegear 3.0
2-3x Dedenne GX
1x Mega Lopunny and Jigglypuff Tag Team GX
4x Cherish ball
one sword and shield base set releases there will be more new staple cards so here are some of them
4xProfessor magnolia
4xQuick ball
4x evolution incense
4x Rotom Bike
4x Fine band
I probably forgot some staples but those are a lot of them
Buy your cards from tcgplayer.com because there are lower prices than sites like ebay and DO NOT BUY FROM TROLLANDTOAD I HAVE GOTTEN LIKE 3 DAMAGED CAEDS FROM THEM.
limitlesstcg.com is a great place to find decklists and results from recent tournaments
I hope this helps