I've never really thought of a limit on the fees specifically, just on the total price.
And that limit depends on the band and the venue.
But to put it simply, £60. I've broken it three times. £65 twice for Black Sabbath, and £70 for the Concert for Jon Lord next year.
Within that price I don't care if it's 5% face value and 95% fees or vice versa.
11/4 Robin Trower
3/5 Maiden United
7/5 One Machine