No, the 720p cost barely a dollar extra. The profit margin isn't so great on the 12" MacBook and the bean counters make the decisions at Apple. More expensive components like the cost of the display and added expense for the new keyboard all eats into profit margins in the eyes of the accountants.
Of course I am being a little facetious but this is the reason why they gave some 2015 tech some pathetic components. As ever with Apple, it's all about money.
Obviously this must be the case. I mean just look at all the places that Apple scrimps on the quality of parts in their devices. I bet they could save a bit more than a dollar by using something cheaper than aluminum for the construction of the device.