You can use eCos to develop products without paying anybody anything, no matter how many developers are working on the product or how many units you expect to ship. This applies not only to the core system itself but also to any changes anybody makes to the core system.

A key aspect of the eCos public license is that it is intended to protect the eCos user community as a whole. It should prevent any company or individual contributing code to the system and then claim that all eCos users are now guilty of copyright or patent infringements and have to pay royalties. It should also prevent any company from making some small improvements, calling the result a completely new system, and releasing this under a new and less generous license.

The license does impose one obligation on application developers: you must include a notice with your product indicating that it makes use of eCos.