The crime is actually a scheme to get rid of the Red-Headed League's namesake, a league of red-haired men that was created to keep one particular red-haired man, Jabez Wilson, occupied and out of the way. The scheme involves using the league's funds to dig a tunnel under a bank, leading to a robbery.
The story focuses on Sherlock Holmes's deduction and investigation to uncover the scheme, rather than the amount of money involved.