How much money does Michael donate to Oscar's nephew's charity, not realizing it is a walk-a-thon and the amount is per mile in S1E4 "The Alliance"?

In the hit TV series, The Office, Michael donated an amount of $25 to the charity of Oscar’s nephew. He thought that he can donate any amount so he pledged to give a $25 not realizing that you will donate any amount per mile and sum it all up to get the total amount he would need to donate.

Everyone who heard about his donation was awed and he did not realize why until Jim, his friend, explained it to him that is not a straight-on donation. He wanted to change his donation to a smaller amount but since it is charity, lowering the donation is like stealing. It shocked Michael that Oscar’s nephew finished 18 miles in the walk-a-thon last year.