Originally Posted by
mercator
You may not need the new derailleur. I'm running a 12-30 cassette with a standard cage duraace rd, had to adjust the B screw all the way, but it works. That would be the cheapest option.
I have seen it recommended that one reverse the B-screw to gain even more range. If it doesn't work, no loss no cost, and if it does, it saves the cost of a new derailleur.