Hmm. I think if properly done can cost up to 30k. depends on what you put inside, covering of walls, laying of floor, insulation works, AC, socket, fan, lights, fresh coat of paint, and most importantly, a staircase. If staircase can source from china then your cost cheaper la. xD haha.. Most people make it as storage area, as its super hot as many /k mentioned here... Oh yeah, need ventilation if wanna convert to actual room, may need an approval also so the officers wont bother your work. If the approval is not approved, gg lo