Thank you Richard for the reference.
And I would first note what Fred said.....There are thousands of existing designs in the world now, stock plans available at a fraction the cost of new custom design. Consider finding one that's close to what you require, and then having it modified to suit, either by the original designer(best), or some other qualified individual.
Cost should not be the main factor in choosing a designer, though it often seems to be. Better to choose a designer who you can communicate with, who is familiar with the style and construction method you require, and best of all a designer familiar with your chosen builder.
As to design fees, it can vary wildly, depending on who the designer is and how much work he does. $10-15k would about the minimum fee in North America, $35-40k for a well known higher end design office, and on up for "superstar" designers. At the bottom end it will cost $1200-1500.00 for each drawing, a fourty-footer might be covered in 8-10 sheets, and a sailing rig or centerboard will be more.
The design fee is a small fraction of the overall cost, yet it will go a long way in establishing the finished and lifetime value of the boat. More or less you get what you pay for. If you buy a custom design from Bruce Roberts or George Buehler it will have a very different value than if you buy a design from Bruce Farr or Ron Holland.