Person1 : "Hey, would you like to go out for a drink? "
Person2 : "Sure."
Person1: "OK, cool."
<a little while later, in the pub>
Person1 : "Well...your round then."
Person2 : "Ok....how much are rounds, usually?"
Person1 : "About £40."
Person2 : "Oh. I've got eleven pence and a piece of string."
Person1 : "Ah...all your money tied up in shares?"
Person2 : "....I've got a housing benefit claim I'm hoping to come through next week if that counts."
And so on.
And while this wouldn't be problematic with two people who were good friends, if it was with someone you've just met it might be a little awkward. Speaking from my limited experience, hanging around with people who are significantly better/worse off than you _can_ be dull (again, if people don't know each other well).
Examples include walking home three miles because the person you're with is too much of a spastic to let you pay for a cab or conversely drinking glasses of water very slowly so as to avoid needing to buy another drink (which would mean not being able to pay for the meal).