foundation
Foundation makes your skin look perfect. It hides your skin imperfections, evens up skin tones. The right foundation makes drab skin look radiant.
What works for your best friend might not work for you. The foundation has to match your skin tone. Obviously if you are fair and she is dark, you would use foundations in different shades.
The foundation has to suit your skin type. Oily skin does best with water based foundation while dry skin would do better with creamier foundations.
Nowadays, foundation is also skin care. The best foundations often include ingredients found in regular skincare that treat your skin.
Another thing you might want to check out is whether the foundation has sunscreen. Some foundations have spf as high as spf 15 to protect your skin when you are out in the sun. A boon for the outdoor types.
If you are into water sports, or are going to sweat a lot, then a water proof foundation might be for you. Be aware that some, water proof foundations might irritate the skin.
Personally, I hardly use foundation as my skin is so sensitive, I get a break out when I try any foundation other than payot or chanel. If I do use foundation, I only use the very best. For these, I go for the stores listed here , and use the discounts available for the cheapest price possible.
Looking for the cheapest foundation product online?
This is what I've found.

Bourjois Plus que Parfait Compact Foundation SPF 9 - #46 Hale 10g/0.3oz
Your Price $13.00

Bourjois Le Nu Vous Va Si Bien Fluid Foundation SPF 12 - #58 Moka 30ml/1oz
Your Price $13.00
Most Discounts
If you want value for money, (I know I do) here are the ones with the highest discounts as a percentage of the original price.

Von Berg Lavish Illumination Foundation - St. Tropez 30ml/1oz
Retail Price $53.50
Your Price $16.50
Your Discount 69.16%
Highest dollar savings.
If you want to treat yourself to something special. Go ahead. With these savings, these luxuries are finally within your reach. This is where you can find the best dollar savings online.

Sisley Phyto Teint Perfect Compact Foundation - # 05 Golden 11.5g/0.4oz
Retail Price $123.00
Your Price $61.00
You save $62.00