It hasn't quite made it into the dictionary yet, but galacticos has at least got its own Wikipedia page:
Galacticos (Spanish for galactics) are expensive, world-famous Real Madrid football players recruited during the "galacticos" policy pursued during Florentino Perez's presidency at Real Madrid, where in his first tenure he purchased at least one galactico in the summer of every year.
It is possible that the term did originate with Alfredo di Stefano and co in the 1950's, but the following slides focus on the players signed since the term came into widespread use across Europe.
All fees are taken from transfermarkt.com, and, although playing a major part in the ordering, the price of the player is not the only feature used to rank the signings—that would be too easy, too laissez-faire and wouldn't account for inflation.
Also take into account this is about the galactico status when they signed—not what they may or many not have grown into.