Converting a flatbar bike to drop bars requires a new bar, probably a new stem, new brake levers and shifters. You will also have to replace the cables and after adjusting the bar and lever position you will need to wrap the bars in bar tape. Figure atleast $200 to have a LBS do it, probably more.
You are usually better off buying a bike so configured from the start.

Many commuters run single speed or fixed gear. These drive trains reduce complexity and increase reliability. You don't need shifters or derailers which can simplify switching to a dropbar. However you only have one gear and it will require you to get out of the saddle to climb. I love my fixie but they are not for everyone.

Craig