Will it save me money?

The short answer is NO, in most circumstances it won't save you much if any money, BUT it should make your money go further. If money is a major concern then using an interior designer may not be the right choice for you, even with the most inventive and carefully budgeted plan, creating a fantastic design will involve a not insignificant cost no.

In more detail, it can save you money by preventing mistakes down the road (if they were going to happen by buying the wrong items or making design errors), but most people who come to an interior designer want the very best design they can get, so will tend to spend the same budget as they would have anyway, but get more for it.