i gave the scientist 1200 gold and now i get 20 gold every day???? I did the math and its gonna take 60 days to just repay the money??? Do i get more money later on????

You get different amounts of gold randomly at the end of the day. 20, 60, 150. Each day's end does not guarantee any of those will happen. Sometimes, you can get all three or two at the same day's end.

I've only gotten 20s and 60s so far, and if my counting's correct I've only just made back the 400 coins I spent on it.