Neeps are great for sheep, we grow about 8 acres of them a year to feed the ewes on the run up to lambing, we just strip graze them and only allow them access to the neeps for about 6 hours in the day. Make sure that when they are on the neeps that they have an area of grass or something to run back to as it can get real muddy if they are left on the neeps all day.
Prices vary greatly depending on location etc, but I guess as long as you and the grower can both agree on a price that you are both happy with surely thats all that is needed.
Good luck