Hi Piggy. Soay sheep are the most primitive sheep in domestication I believe, remnants of a flock from the St Kilda island group which may well have had no introduced blood ever, so they are similar to the earliest domesticated sheep whose bones have been found on early inhabited sites.
How much to pay depends of course on whether they are registered or not. Unregistered Soay are not worth much at all
But a good reg ewe could be
up to £60 if she's young and in good health (a bit more in England). How much to add on for the lambs would depend on their sex - more for ewe lambs, say £10-20.
There are lots of tales of Soay being escape artists but we have never had that problem. It will depend on where they are from and how they have been cared for up until you acquire them. If they have run as a semi-feral flock they could be quite wild, but if they have been used to people they could be more friendly than most other breeds. Good fencing is necessary - they will go under as well as over if they are not happy.
For gathering, Soay prefer to be led than to be rounded up by dogs - they don't flock like most other breeds. They are intensely curious and have very well developed individuality, neither of those characteristics being typical of commercial type sheep. Ours will all eat out of our hands and like a little bit of digestive biscuit now and then
Feet are generally good, having dark hoof. They need all the usual worming, vaccinations, anti-fly-strike etc that other sheep need, and contrary to some tales floating around they do need to eat - some grass and some browsing, with ad-lib hay and a small amount of concentrates in winter. The fleece will 'roo' off on it's own or you can shear them or roo them by hand. They have 'scadder' which is a mane and hair along the spine, which stays behind to keep them a bit warm after rooing.
Soay hoggs can be a bit delicate with a tendency to die in their first winter - doesn't often happen but keep an extra eye on them. After that they are very hardy.
Last but definitely not least the meat is very special. They take a couple of years to get up to slaughter weight then give a small but very tasty carcase, with close grained, tender meat, which is full of flavour and usually wins over those who say they don't like lamb.
If you want something different and are prepared to love them - go for it.