I met Tinashe for $80 at her show last year, but considering she isn't that big yet, it was very reasonable. I think i'd pay no more then $500 to meet Rihanna. Maybe a little more then that but not definitely not in the thousands. Mine would have to include a front row seat and picture.
I'd have no issue paying $1,000 to see one of my faves even though most have passed their prime. Anyone else, I'd pay no more than $200 to see them, depending on where they are in their career. If they are A list, $200 is fine. If they are on their way out. Maybe $100 tops.