I have bought and sold at York rare breeds since the first sale. As inferred, you will not get a brilliant price, on the whole, for sheep or cattle unless they are top quality and registered.
There are of course exceptions, and some of the poultry fetch ridiculous prices for poor birds (lice, scaly leg, diarrhea etc) because some of the buyers just see something that's pretty and don't know any better.
However the sheep and cattle buyers tend to be more discerning and know what they are looking for. (Usually a bargain!)
To maximise the price you get you need to present your animals to the best of your ability and stand by the pens when prospective purchasers come round so you can tell them the good points - ie they're tame, reared twins last year, off a famous sire, or whatever. Also, be prepared to go in the ring with them when they're sold - if you can't be bothered to stand by your animals when they're sold then why should anyone be encouraged to pay a decent price for them?
Yes you do get runs of ewes going for £15 -£20 each. Normally when someone has flooded the market with a batch of mediocre looking animals with no useful information accompanying them, and no vendor in sight either. Would you pay anything over the odds for such as these?
So you're planning to put 50 ewes in then Coximus. I would actually count that as flooding the market! I would actually pick out my best 6 and go to town on them - make them look as good as you can, put an amazing description in the catalogue and hope for the best. If you get a good price then it was worth it! (Or maybe not, depending on how you cost your time!)
If you don't then at least you haven't wasted your time bringing the other 44 in.