I have been CHRing for only two months, so I think more experienced CHRers will give better answers, but here is my 2 cents:
You are taking 6.67 rolls to find a wheat (7.5 wheats per box($25)), my averages are 5.6 machine rolls to find a wheat, 3.9 hand rolls to find a wheat... So for me hand rolls give better hit rate, being said that standard deviation of hand rolls is almost 10 times of machine rolls - so hand rolls are all over the place sometimes very good sometimes very bad, while machines are more consistent, so I try to mix it...
Going back to your original question, I think your stats (7.5 wheats/box) as well as mine (11.1/box) are on the lower side... I do not know the official stats but with eye balling others findings I think 14-18 wheats per box is the average... My best guess is location is a major factor in the hit stats...