Not bad for an approximation off the top of my head. I would expect £15000 of sales in terms of programs, merchandise and food anyway offsetting the difference with 5k extra people.
To be pedantic, at the rates I described 20833 would need to be the attendance to match using the same assumptions you did in your above calculations. And as a point of note, our average attendance last year was 22,253. Which would (again, using the same assumptions) generate a profit of £25560 compared to a crowd of 15k paying 25 pounds per ticket.
Out of curiosity Flyer, do you work in finance?