The mods you're referring to dleefoto can be done for less than $100 total. Call up your local sign shops and ask how much for a 4'x8' sheet of coroplast (corrogated plastic). . My local sign shop will sell me a sheet for $20. That will be enough to do your rear wheel skirts, front grill block, and most if not all of your underbelly. You'll need to get some hardware from the local Home Depot, and spend maybe $20-30. It all depends on how clean of an install you want to do and how much you care about the body of your car. I see you have a 2010 Ford Focus, so I'm assuming you're going to want to do as little damage as possible. With my Geo Metro, I have plastic bumpers so I don't care about them much and don't feel bad screwing into them.
As for cost:benefit, that all depends on how much you drive. I drive about 30k miles per year, so for me my mods pay off fairly quickly (well, except for the smooth hubcaps). If I save $0.005 per mile for a mod that cost me $30, it will take 6000 miles for that mod to pay for itself (that's going from 45 MPG to 48 MPG). Your cost and mod effectiveness will vary depending on how you drive and how you do your mods. Read up on the 65+ Efficiency Mods and the 100+ Hypermilling Tips links that are at the very top of this page. Those will be a good start. Remember, the cheapest mod is to simply change your driving habits to be more efficient. In my 94 Geo Metro that I had I could average 33 MPG, but with a change in driving style I once got 49 MPG out of it.
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