Well, to begin with, Salisbury was a cathedral city, which made the Earldom fairly decent as a fiefdom went. But the key to Richard’s wealth was that he married Anne Beauchamp, who was daughter to Richard de Beauchamp (Earl of Warwick) and Isabel Despenser. Isabel Despenser was the daughter and sole heir to the fortune of Thomas le Despenser, Earl of Gloucester, a descendant of one of the most notorious families in England who were so grasping and venal that they provoked the entire nobility of England into a civil war against Edward II.
So Richard didn’t just get the title of Earl of Warwick, he got a huge swathe of land across south and central England and parts of Wales.