"Best Guess" cost of new megayachts

Hello Everyone:

I have been reading the posts for a few months and finally joined.

I see advertisements in yachting magazines all the time for used megayachts and the prices are sometimes given.

It appears a 50 meter megayacht can be had new for about $35 - 40 million USD.

What would a 280 ft. megayacht cost? Would you just double the price of a 150 ft yacht to arrive at the cost?
Lady Moura is one of the most beautiful yachts I have ever see. What do you folks think she would cost today?

Any thoughts? Sorry if this first post sounds naive.

Pinellas Park, FL