Yes not allowing for special deal's from promotions i do think the extra coding would have been put into the end price.
Ask yourself this , As a dev basicaly you have to for a market share code in DX,(which is why u dont see mantle only games out there) If you also have to hire people for mantle and/or any other API's gpu venders want done wouldnt you add this to the final cost or would you simply do it for free?
I know as a business person myself i'd be adding it
I don't think one guy and a few months is going add to much if anything to the cost.
At the end of the day its those buying, not selling who dictate the price. if people are happy to pay it they will, if not....