Again, it's not solely about money, as evidenced by Atletico. It's about spending right. Case in point - Barca's board. Aside from Mascherano, Suarez and Rakitic, they had the prestige of Barca AND the money to spend yet they've generally let the fans down when it comes to rebuilding the team (specifically CB and a successor to Xavi).
City and Arsenal's money will allow them to stay competitive. That much money means they'll probably never fall outside the EL places. However, building a superpower requires much more than money. See United 06-09 built on bargains like Vidic, Evra and Ronaldo, or Pep's Barca built on homegrown talent mixed with smart purchases like Toure, Villa and Keita.
Agreed that transfers are 50/50 propositions most of the time, but it's no coincidence that the players United and Chelsea acquires regularly end up being worth the money. Great decision making will turn that 50/50 to a 60/40, or even 75/25. Going back to the Atletico example, look how they've never fielded a team that didn't have a great scorer up front despite having a very limited transfer kit per year.