Most versatile foundation

Hi, I'm just starting being a freelance MUA and I wanted to know what was the best, cheapest foundation for all skin types and the most common shade in them for like pale, tanned, black skin etc, thanks guys! 

Your best bet is to get foundation palettes, as they are more portable and efficient as opposed to stocking your kit with individual bottles, pans, sticks, pots, etc. also no one really mixes foundations anymore to get another tone. It does happen, but very rarely. As to where to get them, you can try 3rd party resellers like Sephora, Alcone, Namies, Frends. "Cheap" foundations / skin care products, won't move as well on the skin and will look cheap in your final result... Just FYI.

Adding to vogueboy's reply, I think that foundation palettes are what you need as well.

Look into ones by Graftobian, Kryolan, Ben Nye, or Make Up For Ever. These palettes will have cream foundations that you can mix into your client's perfect shade.