I agree, 30 bucks for a mag is terrible, are they made of high grade alluminum? steel? no, they are just ABS mags, as Got Wood said, the R&D process can't skyrocket a mag price to six times the price of a regular magazine (and that's not only for the MP7 but for the PTW, DAS, EVO, ARP9...)
I see this situation similar to the printers, the printer is "cheap" (well, this MP7 is not cheap either) and they get their money using overpriced consumables, in this case, mags, so if people wants to skirmish the gun properly, they will need at least 5 more mags for a total of 6 (at least that's my personal minimum) making you spend 150 dollars more
People will buy it, but I doubt it will be a "hit"