It doesn't really matter who passes data on to the paper does it? It's their responsibility to ensure that it's correct before publishing it
That data may well be accurate. I suspect the way they've calculated the averages (or hope they did this) is from the population of each ticket bracket. So yes Arsenal do start at just under £1k (my gf is a season ticket holder) but I know there are some tickets for the club class area that are £3k or more. Not sounding like I'm defending the data (I've not seen the methodology) but because the averages can't be proven in 10 minutes, it doesn't mean they are wrong. I'd hope this research took considerable time to amass and involved speaking with clubs directly to understand each ticket price, availability, offers and volumes of tickets in each category.