I owned Stillwell financial stock (they were the parent company of Janus Capital). In 2001, they separated. What is the cost basis for each stock?
Unfortunately the information you have about there being a spinoff is incorrect.
Janus Capital (unlisted) merged with Stillwell Financial (listed).
For shareholders of Stillwell, the only effective change is that your Stillwell shares became Janus Capital shares on a 1:1 basis. There were no events (splits, spinoffs etc.) that would affect your cost basis.
Therefore your cost basis is whatever you purchased it at in 2001.
Source: Internal databases as Norgate Data.