I am so surprised at the amount of people more or less saying OP has no cause for complaint and should happily continue to shell out if she can afford it. No wonder there are so many uncaring, selfish people under 40 in the world (and many over, including us olduns). If it had been established from the outset that the monthly visit was to be her treat of a meal out, fine. If the couple even sometimes put some toward it, fine. But the fact that they do not even send a card on birthdays etc I find heartbreaking and points to the relationship being totally one sided. Someone suggested it is not a pay back thing, doing it for what you get back. Maybe not, but surely any child who loves their mother (and father) would at the very least send a birthday card if they cannot afford a present.
I am conscious of the fact that I speak from a privileged position in that my two sons are the total opposite. They have wonderful laid back wives who embrace me as mil and are happy to share the grandchildren with me, knowing how much I love being with them.
When we go out to family meals they absolutely forbid me to even contribute (though I do try and often give the grandchildren generous pocket money instead). I go to their houses for Sunday dinners and they also come to mine in turn, the oldest 1 hour away, the youngest 15 minutes.
I am comfortably off, though not well off, but we all give each other lovely presents although they tell me off for buying them things or taking a bottle of wine to dinner (nicely told off) but always bring flowers to me. My sons tell me a card with loving words means everything to them and they feel I deserve whatever they give me for all I gave to them as they grew and during early years of their marriages.
Surely this is how it should be? I know family dynamics differ but if a child sees you as a money machine and never gives anything back (a bit of gardening, or D.I.Y.?) then how is that a healthy relationship. The OP is definitely being taken for granted, more than that she is being used and she must insist on it ending. Don't worry they will not cut her off, it seems they need her more than she needs them. In fact, I have a feeling they will have a new respect for her, rather than seeing her as an easy target for free monthly meals. I would cook a meal and invite them with the proviso they turn up with dessert (a cheesecake is £1 in a lot of stores). If you act like a doormat, everyone, including family, will walk all over you.