And what kind of issues does it cause?

http://www.dailymail.co.uk/femail/article-2296041/How-feels-best-friends-earn---Its-topic-women-shy-away-Here-break-taboo-So-friendships-survive.html

My best mates in London all earn more than me, and it definitely causes some issues. We used to meet every week for a meal and go to Spoons or places where we'd get vouchers for, but more and more they want to go further afield to more expensive places, and it just starts to stretch what you can afford. I don't resent them doing well, they do jobs I would never want to, but it's a shame when you start to drift apart because of cash.