how can it range in that big of a price? what are the differences?
You asked for the price of a DEER or ELK head. An elk head will be hundreds more than a deer head.
I've seen deer heads range from $300-600 and elk are significantly more.
The range he gave you was between the cheapest deer and most expensive elk. It's like you asked how much it costs to buy a car. While the Ford Focus and Mercedes 600 are both cars, they will vary in price quite a bit - like $100,000.
It also depends on your geographic location and the quality and reputation of the taxidermist.