PUT and Idempotency??

Hi,

Having a problem getting my head around this and wondering if I really need
to understand.
I'm thinking that if my application receives the same input, then I should
always return the same output, but I don't see how that's possible. For
example, if the customer asks for a quote today, I'm not going to send
back the same data tomorrow if he sends me the same data and our prices
have changed since.

My web application is already receiving JSON data via a POST method. It
calculates and saves an order and sends the details of the payment back.
Now I have to create a simulation via a PUT method. The application will
receive the JSON data in the same format and do the same calculation except
there will be no saving of the order on our system, just the details of the
customer request.
So I'm thinking I'll just ignore the fact that the http method is PUT and
treat it as a POST.
Am I missing the point?
Thanks!