You will need planning consent and then there will be ground work to be done such as installation of drainage, you will need a rubble base layer for drainage as well and then there's membrane and surface to be put in, if you want it done properly and to last for years.  Some contractors estimate all this at about £15k or so.  We made a school at our old livery, it was in a little paddock at the back of the house and no-one passing was able to see it so we got away with doing it on the "fly" without the permission (naughty), we knew someone with a JCB who did all the drainage and ground work for £450 including flattening the area out, my dad is a haulage contractor so got us the stone layer cheaply and my brother in law worked for a builders merchants who got us good discount on the membrane.  We surfaced it with easiride from the same company as easibed which is a woodfibre base, it doesn't freeze in winter usually although with the temperatures we had this year I expect it probably did, and it doesn't need too much mainenance, it stays soft and springy and wears well, it cost £1600 delivered from Preston to Scotland and we spread it down ourselves.  All in all I think we spent about £2500 on it, roughly, but we really did most of the hard work ourselves to cut costs.  That was in 2005 and it's still there yet.