What monetary value would you put on a gift card for someone

[DeckHallsWithFIMBOughsofHolly]

who has given you a lift to an evening course over the last 12 weeks - approx 20/25 mile round trip (will not taken petrol contribution).

I am going to buy a Clarks voucher as a Christmas gift for her do you think £20 is ok, or would you give more?

She loves shoes hence the Clarks voucher (as a contribution to a pair she has her eye on)

I think £20 is very kind.

More if you could..but not knowing your financial circs I can't comment really - how much are the shoes?

£20 is perfect.

Thank you - didn't know if it was too mean or not. I have no idea what the shoes cost she just said she had seen a pair in Clarks that she liked. (who knows they may go in the sale!)