This is about birthday presents just as much as Christmas...
I live alone and though I have enough money I don't have any spare. For birthdays I can only afford £10 gift as I have to take the money out of my food budget.
The thing is, my parents can afford to routinely buy themselves things I'd consider a treat e.g. nice box of chocolates. If I give those as a gift I just look mean and miserly. Likewise I've often sent £10 voucher as a birthday gift, only to find out other members of the family have sent £20 or £50.
I just feel embarrassed and rubbish because I can only afford inferior gifts.