I don't think in Sids case he was a designer dog, I think the owners had a boxer and a ridgeback, but neglected to get them spayed and then when the pups came along they didn't think they would be able to get rid of them (being such large dogs) so gave them to the RSPCA.

I personally would always be happy to take on a crossbreed but wouldn't pay high prices, I would go to the rescue centres.