The original phrase is "You look like a million bucks" which typically means that the person is well-dressed or good-looking. So to say "you look like a hundred bucks" is an insult, because just as a hundred is significantly lower than a million, the person does not look as good as they should look.

It's a thinly disguised insult.

Sigh. A hundred dollars used to buy a lot in the U.S. And for those of us who are frugal, it still does. A person who would DISDAINFULLY say, "You look like a hundred bucks," is in a higher economic bracket than many. Of course, as Marcia notes, the statement is actually a clever revision of the original million-dollar expression.