In general I would expect a cross to have less value than a purebred ewe, but if you keep them it may not matter. The question will be what do you then breed to and if it is something different you would end up with ewes of probably less value still. Dorsets are also renowned for being able to breed all year round and to dilute this would also take away from their value. Potentially crossing could increase the vigour of the offspring but this would probably be marginal since both breeds you are considering are of Down type. For producing meat ie the boys no it doesn't matter crossing except to say that the Southdown is a small breed so this will affect the size of your carcases and therefore their value.
Personally if I had Dorsets I would breed them pure and give consideration to breeding out of the normal season - if you have the set-up to produce early lamb then the value of that lamb is more. If I crossed them I would probably use a ram from a breed more known for meat production such as Charolais or Meatlinc. You could use Texel, Beltex or a Suffolk which are more common although they are not my favourites for various reasons.
Value of course what it is worth to you, whether you sell as breeding stock or keep them yourself, or whether you eat your own meat and what you would have been prepared to pay for it. You don't mention what your land is like and whether you have buildings to use, which may have a bearing on your decision. Ultimately what you decide to keep will probably be what you like.