that's like asking how much a car costs!
depends on the size you want, where you are in the country (distance from quarries makes a huge diff to delivery costs), what soil type you have (affects groundworks), and what surface you want - what you want to do on it, and how many horses per day are going to be on it.
when you know all this, i'd ring round some suppliers and get estimates, or ask on here - lots of people have done their own.
good luck!