An interior designer can provide years of expertise, industry knowledge and artistic creativity to the client - some on an hourly fee basis, some on a cost plus/product mark-up, or a combination of both. Know the costs up-front and the difference in billing styles - while the cost plus/product mark-up sounds harsh, in reality it can save you a great deal of money. Some suppliers sell only "to the trade" and can offer discounts up to 50% off retail. A designer may pass this discounted price to their client and add 10 - 20% as their "fee" - and the client still saves 30 to 40% or more!
If you're hiring a designer, make sure the contract clearly states all the costs, including incidentals such as telephone calls, internet search, and travel time. Designers frequently know the latest and greatest products and materials out there, so they're not re-inventing the wheel with each new client - you'll have the advantage when hiring an expert. Get recommendations and referrals and be sure to check references. You'll be glad you hired an expert who can save you time and money on your design project.
For a referral, you can visit the ASID (American Society of Interior Design) web site: http://www.asid.org/referral.htm