Calculate your hourly rate. Your district has this somewhere.

Add up the number of days/hours you're already working outside your contract. Subtract that amount from what you've been paid in stipend. The stipend will run out of money usually in mid-October.

Start putting in requests for extra duty pay, authorized by your HBD.

A stipend is to cover the EXTRACURRICULAR portion of your job - football games, concerts, and the like. It does NOT cover Summer Band (unless the agreement says it does) nor does it cover extra Saturday duties.

They want you there? Pay.