How much for gift card? Please help!

I carpool with my neighbor to send and pick up kids (she has two and I have one). I send them to school in the morning and she picks them up in the afternoon. The afternoon one is more complicated than the morning ride, because sometime she has to go twice due kids have different activities at school. I would like to send her a gift card to express my thanks to carpool with me. How much is the right amount to put on the gift card. We have school from Sept to June (10 months).