Satin Taupe from MAC is one of their most popular eye shadows. When I first started getting into makeup, this was the fist MAC eye shadow I bought. This is a great color for all over the lid or to darken the crease and really great for a neutral look. I'm always looking for good cheap dupes, especially for MAC eye shadows. A lot of online forums and blogs that post dupes for MAC eye shadows say that Iced Mocha from NYX is a good dupe. I originally bought this color without even realizing it was a dupe. In the pan they do look quite similar, but when swatched, they're actually quite different. Satin Taupe (left) is definitely warmer, while Iced Mocha (right) is cooler toned.
Both are really nice, smooth and pigmented and last a long time with a primer. Price-wise I'd say Iced Mocha, or any NYX eye shadows really, are a bargain for the quality you get. Personally I prefer the MAC one over the NYX one, just because it's warmer toned. But if you're looking for a nice alternative, NYX is a great choice too.