Cost/benefit analyses are a favorite of mine. But as the saying goes, garbage in -> garbage out. In other words, you should only use your analysis as a basis for judgement if you captured all the variables that should go in to your decisionmaking.
Ecomodding saves gas money, reduces wear on the car, and it entertains me. It also reduces fossil fuel consumption and CO2 emissions. An ecomodded car can be a conversation-starter, with social and environmental benefits. All of these things add value, and if you look only at the gas you save, you're missing a big part of the benefit.
You can capture the fuel and its CO2 / sustainability impact in this spreadsheet by using a higher fuel price than you pay. I like to add $3/us gal to the price of fuel in spreadsheets like these, but YMMV.
You use a payback period method here. That's okay, but remember that money you put into investments, savings, stocks, and bonds also has a payback period. If the payback period is longer than about 8 years, the mod does not pay. Also, if the payback period is longer than the service life of the mod or car, the mod does not pay.
When Ecomodding is a hobby or a sport, it's fine to make mods that don't quite pay for themselves, as long as you enjoy installing them and driving around with them.