I never budgeted for supporting parents: my mother and both in laws, my dad died a couple of years before I married. Honestly it was difficult for us to budget for ourselves years ago.
They were not rich but better off than us, they helped us out. However I did help in other ways, we were very supportive of them in old age and they knew they could depend on us to do anything to help them.
I am now 61, funnily enough late last night I was talking to my eldest about me thinking of moving house and the subject of money came up. I was reassured that I would never have to worry about it :-), which I thought was very sweet but.....I don't worry about it. I did all the worrying when I was young, am content now.
I often say to people who are dreadfully stretched financially, that it really does improve as you get older. As long as I have enough to be able to employ carers to come to my home should that be necessary, I am content.
There is absolutely no need for my children to 'budget' for me but I know they will support me in other ways and make sure everything is OK.
It's quite a different matter if a parent is really poor, then it is nice if their better off children can make sure mum and or dad have all they need, ie eat well, are warm enough, have the right clothing, help with repairs, take them out sometimes, etc.
A lot of elderly parents are too proud to admit they need financial help and try to cover up problems but it usually comes out eventually and can be sorted out tactfully.