Average gate last season 25k x average cost of £25 per ticket x 23 games = £1,4375,000
Average spend inside the ground per season £2,875000
Sub total = £17,250000
Probably takes a further million for car parking charges.
That is my rough estimate, so give or take a few million, it is definately a lot of millions.
It was £11.8 million, substantially less than your figure of £18-20 million