Yes there are clearly different norms in different groups. Of course there are. And if you can't afford something, you can't afford it. This applies to everyone.
However, I think some people on here are talking about a specific dynamic in their quite intimate friendship groups that have their particular understandings.
I'm talking about the issue more generally - eg. if you are having a party inviting people beyond your immediate 'closed' group with your mutual understandings. Like the actual question posed by the OP who was not sure what to do because her situation is not as clear-cut as "I always out with 6 friends and we always go locally and we always pay for ourselves..."
Obviously we can all only do what we can afford. But some of the attitudes expressed in this thread do come across as a bit clueless - like some people haven't grown out of the mindset of "it's my birthday and is all about meeee," coupled with this cop out phrase "they don't have to come after all," or "why should I pay for what they eat?"
Beyond very close-knit groups with particular understandings, if you, as an adult in your 40s, 50s, 60s decide to arrange a birthday dinner for a slightly more mixed group (eg. friend with partners or husbands you may not know as well; people who have had to travel or get babysitters; people who may not all be local best mates around the table) - then, I'm sorry but in this scenario, you host, you pay. Or don't do it at all.