He is the author who wrote this on his website:

Q. How can I get Neil Gaiman to make an appearance at my school/convention/event?
A. Contact Lisa Bransdorf at the Greater Talent Network. Tell her you want Neil to appear somewhere. Have her tell you how much it costs. Have her say it again in case you misheard it the first time. Tell her you could get Bill Clinton for that money. Have her tell you that you couldn’t even get ten minutes of Bill Clinton for that money but it’s true, he’s not cheap.

On the other hand, I’m really busy, and I ought to be writing, so pricing appearances somewhere between ridiculously high and obscenely high helps to discourage most of the people who want me to come and talk to them.

He’s busy, for sure.  Too busy to agree to every appearance request.  And so he does need to discourage people who want him to come and talk to them.  But does he have to use high prices to discourage them?  He could always just decide how many appearances he is able to fit into his busy schedule and agree to that many, saying no to everybody else.  No need for prices if he is just rationing his time.

But maybe he wants to make sure that his scarce time is allocated to the audiences that value it the most. That’s not greed, that’s efficiency.  Then instead of rationing by saying no, he should hold an auction.  He chooses the same number of appearances as in the rationing mechanism (just based on the cost of his time), but now those appearances go to the highest bidders.

But maybe his optimal quantity of appearances cannot be determined independently of demand.  If the auction fetches a very high price then he knows that the marginal willingness to pay is much higher than his marginal opportunity cost and he should increase supply.  As a result his marginal opportunity cost increases until it rises above willingness to pay and he stops there.  Now he is even more busy.  But that’s efficient.  And the price is lower.

But opportunity cost is a slippery concept.  Agreeing to additional appearances means lower prices.  The lower price is therefore an opportunity cost of the marginal appearance.  When Neil Gaiman takes this into account, equating his marginal opportunity costs to marginal willingness to pay means raising prices.  Now that’s greedy.  But on the other hand any way of increasing consumer surplus necessarily lowers Neil Gaiman’s profits, so its also (Pareto) efficient.