Backtrack to the area of Waterfall where Undyne grabs Monster Kid by the face. Here, there should be a pink blob. This is the Ferry, and you will be using them to farm.

Initiate a conversation with the Ferry. It will say something along the lines of "3 gold for the ferry" - agree to this condition and board. You will promptly be taken to a later section of Waterfall and given 3 gold. Repeat until you are rich, or until you die of consumption