Thus far most of the trade-in options seem to work out like this:

Cost in Hinterland Prosperity = Sale value of output in pennies - 200 - Hinterland Efficiency

So for rough estimates we can treat 1 HP as worth at minimum 1 penny, and thus the minimum EPA as Hinterland Prosperity per action divided by 100, but the markdown when cashing it in will increase the actual EPA by varying amounts, with lower-cost options being more efficient than higher-cost ones due to the discount being additive.