Much more easily, instead of plotting profit versus ad spend, plot (profit - ad spend) vs ad spend. Choose the highest point. If it looks like there might be a

Much more easily, instead of plotting profit versus ad spend, plot (profit - ad spend) vs ad spend. Choose the highest point. If it looks like there might be a higher point not covered by your points, like how in OP it looks like the cheapest ad spend is the best, try going even lower. Instead of extrapolating, test. Extrapolating is much harder than interpolating and often requires some causal or structural understanding.