As soon as the fish passes, you should wrap it in a damp cloth, than wrap it in a plastic bag, making sure you get all the air out. Put it in the freezer until you go to the taxidermist. The cloth will allow for better preservation when thawing.
I was looking around on the internet about a year ago for prices to get my oscar mounted. He was 15 inches, And I found a range in prices from 17$ -27$ per inch.