There is often a price difference, but I've seen the 600ohm at the same price as the 250ohm (Premium) or very close in the past 2-3 months. They're easy to find at $349, but the real deal is finding them for $249. You can easily do that on the forums, and Amazon's had some real nice prices recently. If people can't wait, the 250ohm will probably be easier to find for a lower price. But if you do a little digging you can easily find the 600ohm for not much more than the 250.
For the past year, the DT880 600Ohms on Amazon have oscillated in price. They start at $349.00, stay there for a few days, then slowly climb downward until they're about $250. They might stay there a bit before jumping straight back to $349.00
I recommend installing an Amazon price tracker like the one above to try to hit a lowpoint. I was lucky and got them at their recent low: $243 on Jan 2.