I put a gold, non-oring, 420 RK chain on my Derbi not long ago and was impressed at the quality for the price. They didn't make one long enough (the swingarm is really long) so I had to buy 2 and join them. I think they were like $18 ea or something. It hasn't stretched much, but I've only put about 1000 mi on it - and we aren't talking much power here.
I'll probably buy a RK again the next time I need a chain.