I'd say the amount really depends on the dragon. If you're feeding a decent amount of veggies, then hitting 30 a day might not be reached. I think a lot of the people that say their dragons are eating 40+ a day, are being fed smaller crickets than the dragon can eat.
For instance, feeding a dragon 1/4" crickets, that has a width between the eyes of 3/4" is going to eat a heck of a lot of crickets, since they are small for it.
For my own two. They eat between 18-25 a day. They are approx 4 and 6 months old. They eat crickets that are slightly smaller than the width between their eyes.
As far as their weight gains. At this point, with the 18-25 a day, they are gaining anywhere between 15-25 grams a week.
So I agree, 30+ crickets a day is probably overkill, for the fact that the people doing this are likely feeding their dragons smaller crickets than they can eat.