of all the things to buy, I thought that the property on the moon was going to be an inexpensive purchase (like naming a star after someone and having it registered with the star registry). 

So you too can buy property on the moon for under $20 USD per acre! Here's how ..

http://www.lunarregistry.com/land/

clivebuckwheat1 said:

He seems to spend money like water, buying the old haunt, expensive cars and property on the moon etc. Any thoughts on his net worth.

 

I don't think he spends that much money. What do we know Castle has spent larger sums of money on?

The Old Haunt – went for $150 thousand but after the murder, etc, I guess it was a lot cheaper to pick up.

The generous donation to the Widows & Orphans Fund to pay for the bottle of Whiskey (unspecified)

The Ferrari (although we do not know for how long he's had that one)

Investment opportunity in independent movie to get Meredith back to LA (unspecified)

Dropping $40 000 (DVD subtitles say $20 000) at a poker table and not being able to collect sustential winnings

The $100 thousand to help get JB's killer in Sucker Punch (was that retrieved after Beckett shot the guy?)

But we also know that for writing three more books the paycheck will be enormous, even by Castle's standard, much more than a lot of his books put together ("For that money I write a dozen Nikki Heat's")

I wouldn't want to put a figure on it but I guess Castle's pretty well off to say the least with the bulk of his fortune securely invested and making even more money while he sleeps and a bit of pocket change in his accounts for his everyday spending and the little things in life he likes so much.