Quality, Price, Speed: pick two.
Do you actually want a custom knife, as in, you have input on the design? Because there aren't many good makers out there in your price range with open books right now.
Jason Clark was the first name that popped into my head as he does a lot of flippers (uses IKBS) and is very reasonably priced within your range. Unfortunately it looks like he closed his books recently, until all current orders are complete.
You can find a lot of hand-made knives on popular sites that specialize in re-selling customs but they won't be custom to you, and you'll pay a mark up.
Alan Davis, Chuck Gedraitis and Eliot Maldonado are just a few of the makers that regularly post work in the Exchange here (that make modern-style knives). Chuck is known for his flippers though the other two have a more gentlemanly style.
If you are interested in slipjoints and traditionals then the $600-800 range for customs opens up quite a lot!
Ray Laconico is another great choice but I have no idea how difficult/expensive his pieces are to acquire these days. Three years ago though...