Hi Richard
Cost is only one factor to consider, If you are new to sheep keeping like me. Things to think about are how easy are they to handle, if you wish to breed them, disease resistance and how friendly a breed are. The shetlands are small but always seem a bit skittish. I opted for Ryelands are they tend to stay put more, are very pretty, a little more footrot resistant, very friendly and easy to handle, mine are registered 2 year old ewes in lamb and cost £50 at Christmas. Some went at the local market for £80 last week. Do you want ewes or wethers? whethers are often cheaper for pets.
I got a book entitled "practical Sheep Keeping" for my birthday which is quite useful and I have spent a year helping my farmer friend through the season to learn about vaccination, lambing etc and for me most important was record keeping, because DEFRA are less than helpful when ringing for advice.
Personally my decision was made as I wanted a breed that was small, friendly and easy to handle easy to clip and clip hooves and a pretty breed which were from a regsitered breeder who I can call back if I need to. The Ryelands are also very funny and mine have very strong personalities and mix well with the ponies.
Hope that helps.