I'm not being frugal, I'm just being practical. Foundations are expensive and you want to make the most of it. There are four other ways to use your liquid foundation other than a makeup base.
1. Tinted Moisturizer
When you don't need a full coverage, mix your foundation with moisturizer to get an instant tinted moisturizer. Use a 1:2 ratio (1 dollop liquid foundation : 2 dollop moisturizer). Added bonus if you have an SPF moisturizer.
The liquid foundation accumulated at the rim of the bottle are great as concealers! Don't waste it and wipe it away! Use it as a concealer to cover up imperfections.
Mix a little loose mineral eyeshadow with foundation and you got yourself a free liquid highlighter!
I love Urban Decay eyeshadow primer but that little bottle is expensive. You can use a liquid foundation and set translucent powder directly on the lids for the same effect. For a double duty, you can also use it on your lips to make your lip color pop!