Yep, I think it depends on the circumstances of the people involved. I'm going to a 40th birthday for a friend and she's shouting everyone dinner whereas I've been to an 80th birthday and expected to pay for myself because the host was living on a pension and obviously didn't have a heap of money to throw around. But a little PS to clarify will make sure no-one's nose is out of the joint. When I had my 30th, I didn't have a heap of money but did have a heap of people I wanted to invite so just chose somewhere that had a set menu and said it was $x per head on the invite so people knew what they were up for. Some of my friends were on fantastic wages, others were struggling students so I thought it was important to spell it out plus I gave people the choice of joining me for a beer afterwards if they didn't want to/couldnt' do dinner.