Someone is who is good might also be more efficient at zeroing in on your preferences and coming up with the right solutions, so the hourly rate doesn't tell the whole story!

a designer for what exactly? Interior items?

designer for everything - tile selection, fixtures, colors, etc.

I think it depends on how much help you need...how decisive you are and how much you want to do yourself. I had worked with a fantastic designer before in my previous house, but in our new build I had done soooo much pre-planning and research that I really just needed someone to bounce ideas off. She was great for stuff like how high I should hang the light fixtures and had great rules of thumb for how many different finishes to ha in the room, etc...but the over-all look was all me. If I hadn't been so sure of what I was doing, I probably would have hired my own, rather than use our builder's ID. As it was...it worked out well and saved money.