Foundation:

High-end foundation tends to have better cover and won't make you break out.
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Mascara:
You're supposed to change your mascara about every three months so why spend so much money on something you constantly have to buy?
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Lipstick:
See a color you like at the Mac counter? Go to Target, look for a similar color... and get it for $10 cheaper.
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Blush:
High-end blush has higher pigmentation, which makes it easier to blend with foundaiton.
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