Ask HN: Side project which can earn $400-500

I want to build a usefull small web app on my free time. Last couple of year I got few ideas and found that either they are build by someone already or they are too big to execute for a person on week-ends in couple of months.

Now I am thinking to make someting for which market are huge on the same time there are small to big players already exists like invoicing app. I am assuming as most of the player already making money, I will also make some money.

Now sure if that is the right path