Rare breeds usually fetch less than commercial breeds because they are smaller, less sort after comercially, and take longer to fatten. So I would look up what commercial breeding ewes are fetching - either in the farming press (google it) or look on something like "Preloved", Farmads where you will see a picture of what's for sale. Then expect to pay less.
However, an animal is worth essentially what you are prepared to pay. Portlands are not renowned for having a lot of twins, so may not fetch as much as, say shetlands. But there again, if you fall in love with some particular animals you may be prepared to pay more, for example because they are very friendly and therefore easy for a beginner to handle.
And yes - you can eat sheep meat at any age - just needs cooking slower if it's older.