Avoiding the doctor because of the co-pay and lab costs. Thats being penny wise and pound foolish: A serious illness is sure to cost more than preventive care.
Using home-gym equipment that hasnt been professionally set up. Even if you get the equipment for nothing, have someone check that its safe and sturdy, or you could pay in injuries or accidents.
Buying cheap processed food high in calories and cholesterol. Yes, the burger and fries are cheaper than the sushi or salad, but its gonna cost you in artery-clogging fat somewhere down the line.
Wearing disposable contacts longer than their useful life. You may save a few bucks but end up with an eye infection.
Starting a new workout routine without a doctors OK. A co-pay for a regular doc visit is a lot cheaper than a visit to the emergency room.