My inflation calculator tells me that's £11.80 now. Even the worst seat in a theatre gig, which what I imagine your 77 one was, would be at least £25 now depending on the act.
Manchester Apollo, 29/4/77, seat X18 in the stalls, a good seat! I think at the time of his first tour PG would not have been able to command higher ticket prices. By the time he played the same venue in 1983 (same seat category) the price had almost trebled to £7. I agree that ticket prices are much, in many cases, too high these days, but then I suspect there are many more people wanting a piece of the action these days, plus the actual cost of taking all that kit on the road that really is not needed. I've another ticket stub I've just looked at for Muse at Grosse Freiheit in Hamburg, October 2003, Euro 21 (£15 at the time) That was the time they were playing arenas in UK, with arena prices!