Im software engineer, well not yet. But you can guess i have a good idea on how much time and cost takes to implement a new api, Mantle or OpenGL or anything else does not matter, still i can assure you than implement Mantle will be harder than implement OpenGL, its a new api, that needs training for devs, and if it has low level elements on it thats worse.
And if your game engine was not developed with mantle in mind it will be even worse.
Im not saying Mantle will not be great, but there is a lot of issues with it to implement in a real world, its not so easy as AMD makes you to belive.