Additional Cost for Custom Designed Yachts

All

I am trying to research luxury yacht costs and am paticularly interested to find out the following information from anyone in the industry:

What would be the anticipated cost for a custom yacht over a production line model. I am considering commissioning the design of 3 yachts in the order of 20, 30 and 40m for fractional ownership.

What is the standard commision earned by a dealer on a large yacht I have heard that 20% is fairly standard, does this ring true?

How dificult is it to secure moorings in the Med/Carribean for large yachts what prices can we expect to be quoted for a good home marina?

How large does a motor yacht need to be for it to be capable of crossing the atlantic in relative comfort and safety?

Does anyone have information on the relationship between yacht length and cost it is obviously not proportional - any pointers?

In terms of designers can anyone suggest any cutting edge naval architects - European based ? What would I expect to be charged for inital designs?

Thanks in Advance

Jon