The black tie bit is a fund raising event. City and charity I think, so you would expect to pay a little extra,The extra £30 for the black tie bit of the event seems eminently reasonable to me if that's you're kind of thing:
http://exeternorthcott.co.uk/the-day-we-played-brazil-2
If it isn't then, as exeternorthcott was at pains to point out, there's still the option to go and see the show for the standard price of just £14. Seems there's just no pleasing some people!