Your right about some wives not being thrilled about wildlife mounts. My cousin shot a 13 point buck weighing over 300lbs. 30 years ago, the head is still sitting in the basement.
Cecil what is the average cost per inch to have a fish mounted, and how much of the actual fish do you use in the mount?
The prices can vary depending on the quality of the work, the work load of the taxidermist (A good experienced taxidermist has a large back load and can increase his prices accordingly), and other factors such as the local economy.
I think the average price now across the country for high quality fish skin work (not replicas) is about $11.00 per inch. There are still people charging as low as $5.00 per inch, and I know taxidermists in Washington state, New Hampshire, and New York that get at least $25.00 per inch. Those gentlemen don't get a lot of work in however, but want it that way. Two of them teach taxidermy.
I break my prices up into panfish, musky and pike, trout and salmon etc. as some species are more time consuming than others regardless of the size. I would prefer not to post my prices as I don't think that would be appropriate here as I don't pay for advertising on the site.
Typically a taxidermist just starting out will be fairly low in price until he improves his quality and increases his workload. Be wary though of a taxidermist that is just starting out and can give you a good deal unless you don't care what your mount looks like when you get it back.
Just like anything else you get what you pay for and it takes time to get to a level of high quality work. I've been doing this for 26 years, and although I feel I produce high quality piece of work, I'm still learning and trying new techniques.
Also be aware that if someone attends a taxidermy school that does not mean they automatically do good fish work. Some of the worst work I've seen were people that just came back from a school and put up their shingle. Most of the outstanding taxidermists I know of are self taught.
Darn why can't I answer a question simply! I'm worse than my wife!
I use all the original parts in my fish except the eyes which includes the skin, head, fins etc. The carcass however is removed and no trace of flesh is left on the skin.
One exception is trout and salmon which I use a cast head as the real head is nothing but a grease ball waiting to bleed grease. Even removing most of the head and replacing it with an epoxy isn't a guarantee against grease bleeding. And once you do that you might as well have used a cast head.
If I do a greasy large lake trout or a catfish skin mount in I will also use cast fins.
Advise:
Don't just consider the price. Make sure you're happy with the quality of work of a taxidermist before committing. They are not all created equal. Fish work require recreating colors with an airbrush and in some cases the ability to carve an anatomical body out of foam. Some never get the hang of it or want to take the time to do it well.
Hey for those of you that have not seen it here is a link on the site of some step by step pics of when i did a diorama for SDSU. For those of you that don't know it may show you some of what is involved.
http://www.pondboss.com/forums/ubbthreads.php?ubb=showflat&Number=124398&fpart=1 
