Depends on your goal with the paint, direct and alla prima benefit from the mixing qualities and applications used with cadmiums. If you want just some inexpensive paints in the same hue family check out Hansa Yellows, Azo Yellows, and Indian Yellows (for various versions of Cad Yellow hues); Cad Red hues include Pyrrole Reds, Pyrrole Oranges, and a number of other pigments that can get you close in color.
Opacity is the main difference though, none of these pigments have the same kind of opacity as found in genuine Cadmiums. That is the major reason we still see Cadmiums marketed, they are very difficult to replace.
I visit www.artiscreation.com
for finding out more about certain pigments used in paints (such as cadmiums). It is a very useful site for learning about the many pigments out there, who makes them, what paints they are in, and so on.