It's going to be hard. You'll need narrower (23mm) tires, drop handlebars and road bike shifters/brake levers, and perhaps a new stem.
It just so happens that after the frame and wheels, the most expensive part of a road bike is the shifters/brake levers.
I don't expect you'd be able to do it for less than $300, and then it would be nowhere near a road bike.
You'd be better off spening $600 on a second hand road bike.