i think you need to do what's right for you and your budget- and if they don't like that, tough.
i'm single and childfree and i totally understand what you mean about the expense. It's too expensive for me to keep up with the 'norm'....i have 7 siblings and 10 nephews n nieces, my friends, and then there's the 'office collections' for birthdays/maternity/leaving do etc.....it is impossible to spend a reasonable amount on everyone, i can't afford it. And although i know it's not about the money it really pisses me off that i end up spending more on other people than ever gets spent on me.
i would suggest choosing WHO you wish to buy gifts for and on WHICH occasions - this includes birthdays, christmas, easter, anniversaries etc - and what your budget is re gifts - whether that is a budget per person or an overall figure of what you can afford as a whole over the year.
e.g, you could decide to buy gifts for the children only and have a budget of £x amount from which to buy all birthday/xmas/easter gifts from. Or you could say ~~ birthdays - kids only, christmas - everyone.
personally, i wouldn't feel comfortable with nor want to feel forced into paying birthday money for someone who has passed on - it's fine if the parents want to do it but it's not alright to expect other people to do it. i have a charity of my own choice who i choose to support so when anyone asks/expects me to cough up for theirs i just politely tell them i can't afford to support any others beyond the ones i'm already committed to.
once you've decided then let the adults know. you don't need to go into detail, just say that you have to work within a budget and these are the people/events that you will be concentrating on. If anyone gets stroppy just look them straight in the eye and tell them you've made you're decision and you don't need their approval for it.
i know some families where it's only the children who get gifts, the adults don't. Remember....gifts are not meant to be compulsory despite what the social norms dictate.
i find that if i plan in advance i can save money in the long run, i.e, i buy wrapping paper, cards and gifts that i know i can give to someone etc during the sales. Boxing Day and New year sales are the best, i usually get about 90% of my stuff then for the coming year seeing as i only buy for the kids so i already know what's suitable. my budget is £10 per person max given their ages, as they get older i might have to increase it a little. i've already decided the regular presents will stop once they reach 16 and after that it will be for special birthdays/occasions only.