Several "real" taxidermists in my area doing them for 125 to 150 for deer and 175 to 225 for elk. The best one I know in Wyoming has a 48 hour turn around during hunting season. He has a 3 person crew and does an amazing job. To the guys scoffing at this, frankly you don't have a clue.
If you are just starting out, focus on excellent quality and don't overbook or cut corners. Build a reputation and you will make your profits in years 3+ after you build local trust. Imo, you should do 30+ heads before ever charging for one. Local meat lockers can get you heads from smaller animals hunters don't care about. Make sure you are good, and are confident in your craft before taking someone's money. One bad job can ruin a taxidermists career. Good luck.