I expect to pay £50-£60 for pure bred rare breed weaners to rear for meat, quite a bit more for breeding stock.
I guess you will get more, leaner pork faster off Pietrain than OSB, so economically it may be worth paying more this end of its life as you'll be saving 4lbs per pig per day for a month or maybe more at the other end of its life plus possibly getting more meat from the carcase. I could argue £20 - £30 more per weaner on that basis - not sure how to quantify the carcase difference, I love the OSB meat!

If you're buying 8 week old weaners now, the Pietrain could be ready by the end of August, possibly sooner; the OSBs probably would want to go on into Sept/Oct - perfect timing for any dry/air curing you may want to do...
Of course I am going to say everyone should start with OSBs - they are lovely, friendly, easy-going, charming, entertaining, and don't get too fat if you don't feed them too much.
Who could resist....