but cheaper than a new bike! £800 ish maybe? Mind you my mate just spent that after his timing chain went and needed a top end rebuild and he used a back street mech. That didn't involve splitting the cases like you need too for the sprag either.
IMHO, dealer for the parts and reputable mechanic to help reduce the labor rates.
If your sprag is gone though what are your options? Throw the bike away? Get in there yourself? (You'd have to be pretty confident in your our abilities for that) Not many people would ditch their bikes with a nice T3...... a ratty one is a different story though!