You need to work on your arithmetic.
If he bought it for $100 and sold it for $200, his profit is 100%: (200-100)/100 -1 = 100%
If he bought it for $0 and sold it for $200, his profit is infinitely high: (200-0)/0 - 1 = infinity. Actually, it is undefined since you cannot divide by zero. Just do the limit as the purchase price goes to zero.
Well, you’re right. I should have said it was “all profit.” However, since I followed tradition and did not read the article before posting, I didn’t catch that he had put in some capital to renovate the house.