It's worth whatever the market will pay for it, and right now, that is out of control.   
 
What is the point of the question, though?  Is it an academic/fun exercise to try and isolate the "value" of lake access over and above just the house,  or are you trying to understand what lake access properties are selling for right now to determine if one is within budget?  If you're a buyer, a realtor could certainly provide you with the data on what properties are actually selling for.  If it's just a "fun" exercise, you could look at data on realtor.com or other RE websites to see what houses with lake access have sold for, and then compare that to similar sold properties that are not on the lake in a neighborhood somewhere.
		 
		
		
		
		
		
		
		
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