Zero. All this "HD games are expensive" is a moronic and untrue narrative that handheld fanboys like to tell themselves to justify their bottom shelf hardware.
This depends how on you are defining HD. Yes, taking a game from PS3/360 levels to PS4 Pro levels in graphics is ridiculously expensive and time consuming. Open up a 3D modeling program and just try making textures, models, and animations on the level of God of War, Witcher 3, or the latest Forza.
Taking a game from 3DS levels to 2007/2008 console levels isn't nearly as hard though. Especially not if your art direction is already cartoony.