This is a string rather than a number "0.07%". You can't multiply a string
times a number.
Try:
=[PowerBallTicketsSold]*0.0007
Make sure the name of the control is not the name of a field.
if commission is 7 percent then
=[PowerBallTicketsSold]*[CostPerTicket]*0.07
if commission is 7 one hundredths of one percent then
=[PowerBallTicketsSold]*[CostPerTicket]*0.0007