At the end of the day they will charge what they think the market will pay. The Lady Gregory isn't exactly huge, and so the pricing is probably designed to reflect its capacity. Some people will pay a premium for things like this, just as some people will pay 1000 a night for a "suite" onboard! Compared to many places in London or Paris, 60 is a bargain!!!
As others have pointed out, there are other (more reasonably priced) options.