I think a great opportunity could be the FIRE community. I think a lot of them think in terms of "if spend X amount I have to work Y more days before retirement". At least I try to do that. The problem is I don't really know how to calculate Y, if your plugin could translate the dollar amount on the checkout page to the number of days I have to work extra that would be really cool.

This of course depends on my savings rate, current assets and expected return on investments.

Either way, cool stuff!