some of my best paid work is on £5 terreaced, can do nearly ten in an hour, compared to bigger houses at 10 -15 but might only do 2-3 , so average is an unfair reflection really, a commercial company might have just offices and average 100 a building, but its no reflection on whether they are priced right or not, so i think average just shows you clean big/small buildings, not how long it takes, or how long you work, many will do £5 houses, but work all day and do nearly 200 in a day, others with 20quid houses might just do 5 and go home!!!
Keep your prices high as you can and work hard, only way to make money!!