All alarms must be maintained annually wireless systems more so due to the reliance on batteries. The idea of changing the batteries yearly is to prevent you having any battery faults occur at 2am or false alarms whist out. The costs you have been quoted sounds correct (my firm charges nearly £10 per battery on the same type of system). You could do it yourself but you will need to know the engineer code which is unlikely.
Don’t forget the cost mainly pays for a experienced competent person to inspect a system which protects your house & possessions.
whoa! £10 per battery