I can see the logic behind your thinking mcongosto. However it is simply not possible to bulk buy and get a better price because if you would buy themes from different authors then who's theme would be sold cheaper?
To put it short, bulk buying could be made possible amongst one authors work, but if you buy multiple's author work then it simply would be to hard to arrange which author recieves less for his theme.
I do agree with Chapterthemes though. The themes here on Themeforest are of amazing quality and are worth far more then the price of a single download When you purchase them and use them for your clients you will probably get a good profit out of it, regardless of the price you paid here.
This is a much reasonable explanation to why this might not work. However, the extra discount should probably come from TF's cut, so I don't know why you designers should worry or get mad about.
The templates might be worth $1000 but they are aslo sol many times, at leat the good ones. It is still not enough? you still cannot make a life out of this? that is something completely different to what I'm saying, and if this is the case the WHOLE model should be revised, because if $40 is to little for for you template, 10% more or less won't make difference.
For example, Template Monster, wich i think is a TF competition, gives resellers 20% discount on every template.
Or even better: the same ENVATO network gives a WHOLE 30% commission to affiliates. Who pays for this commission? Envato does, not designers, right? this is exactly my point.