Are you talking about do-it-yourself, or sending animals to a taxidermist? I have several ducks, geese and one deer mounted. It can be expensive, but I have used a guy for many years that is much lower than the market rates. He is a couple of hours away in a small town, but I pass through there occasionally, and he comes to my town occasionally so it works out. I would say shop around. In a city, the price will be very high. If you can get to a smaller market, you may get a better deal.
My sister has a mounted buck trophy (typical head & antlers on a wooden mount) on her living room wall. Very unusual in a private home in England. You find a lot in manor houses, castles, etc., but not in a common home. Some kids came to visit and were staring at the trophy, and she casually told them that the rest of the animal was on the other side of the wall. Some of the kids actually went to see.
My first 10 point white tail is a head and shoulder mount. Looking at it I can still recall all the details and excitment of that day. Cost then by a local woman licensed taxidermist was $40. It would be closer to $600 today.
The last thing I had mounted was a whitetail buck in 2007. Cost me $250 at my guy. In the city I live, it would have run me $450-$750. More in bigger cities like Houston. The guys I use does great work, but lives in a town of 4000 people two hours away from anything bigger. He has to work cheaper to get business.