I've had a horse shipped from San Antonio, Texas to Fort Collins, Colorado for $1300. The same horse is getting shipped from Fort Collins, Colorado to Aiken South Carolina for $1800, the trip is 625 miles longer than the one from Texas to Colorado.
It depends on the company, the size of stall you want in the trailer, if you're having items shipped as well (i.e. a saddle, some grooming supplies, etc.), if your horse has special needs that need to be taken care of, if your horse is a bad loader, if you have to have an overnight layover, etc.
"Always be yourself. Unless you can be a unicorn. Then, always be a unicorn."