Check out
this thread and
this thread.
One of the more economical ways to convert from flat bar to drop bar is to use bar-end shifters instead of shifter integrated with the brakes. The most affordable option may be to use friction bar end shifters such as Suntour Barcons. They ratchet in the pull direction and are friction in the other direction, providing fairly precise control of position. If used with a system that is otherwise indexed (ramped cogs, etc.), the performance can be almost as good as indexed shifting.