How does $15 grab you? Also very useful for bottom-bracket lockrings and other applications:

http://harriscyclery.net/itemdetails.cfm?ID=1390

The more I look around, the more I am convinced that Shimano has gone back to it's old trick from the early 1980's - making proprietary parts and the tolls to work on them. Here we have another golden ($$$) example. I mean, why does this cog have this lockring? Is it absolutely necessary? Or was it created, alongside with it's $50 tool, as a money-maker?

Back in the early 80's Shimano sent a representative to address a trade-show for store-owners and mechanics. When the rep. from Shimano approached the podium, he was booed and objects were thrown. The cost to store-owner/mechanics had reached $400/year for the latest Shimano-toolsets. Needed to work on Shimano parts/components. Shimano finally got the message.

Maybe it's time for a repeat...