The cost all depends on what you're willing to accept controls wise. Bars and likely stem with tape of course are the obvious first items. Then there's the brake compatibility with the drop bar levers. If you use bar end shift levers or friction ones then a low cost drop bar brake lever that's compatible with the MtB disc brakes will do. If you want STI brake/shifter levers then add a set of compatible disk brakes. last is the saddle. As one changes the way the crotch interfaces with the seat sometimes the seat will be wanted to be different.

So with this list and guide one can add up the costs then add a forgot about * aspect cost, just in case. Andy.