RM and Barca may be miles richer than the other La Liga clubs, but it doesn't guarantee either of them winning the league, as last season's Atletico Madrid title win attests :)
RM are the biggest sports entity in the world, and it is their ethos to buy the best (most noteworthy) player at the time of the summer transfer window to appease the Madristas. Liverpool of the 70's/80's had a similar idea. Buy one player to perk up the squad and stiffen competition for a start...