Interaction designers, on the other hand, care exclusively about how things work. They spend so much time caring, in fact, that they tend to drive everyone around them crazy with their complaints about everything from the design of refrigerator door handles to iMac mice. Most interaction designers, unless they are trained in graphic design, absolutely depend on their graphically-talented counterparts. Likewise, most graphic designers and industrial designers have enough sense to depend on the talents of interaction designers and ergonomists. A few just must strike out on their own, usually in both senses of the word.
Interface design is usually very, very expensive.
That's because interface design is usually done shortly after the product ships. If, however, you are clever enough to do it at the beginning of the project, it can save you time, effort, and lots and lots of money. A good designer can develop a competent, implementable design in short order. After sufficient iterations, if you have have carefully chosen your user population, and if you have been honest with yourself about the outcome of the tests, you will achieve a successful design. The savings brought about by such a procedure begin to pile up quickly.