The cost of supporting a specific platform for a specific duration is very hard to estimate. There are three main types of costs. The most obvious cost is the direct hosting costs for servers and bandwidth. The type of cost is the man hours of administration and maintenance of the buildfarm and management of the distribution. And the final type of cost is the developer effort to maintain all released packages to avoid bit-rot.
Unfortunately the ability to measure the costs is correlated with the magnitude of the cost. The direct hosting costs are small compared to the cost of having a few people maintaining the distro. And that is a much smaller cost than the collective work required by all the maintainers.
Other factors to keep in mind are startup costs vs maintenance. There's a very large effort to kick off a new distro up front to fix all the issues which occur from changing dependencies upstream. There's also correlation between distros, sometimes changes can easily port between similar architectures, but sometimes they need architecture specific patches.
Also relating to extending support for EOL'd platforms. The cost goes up significantly as the upstream dependencies become EOL and no longer get updates and patches in parallel. (For example there's an issue with arm_navigation_experimental in groovy which only manifests on Quantal. To fix this will likely require a patch to work around the lack of updates upstream because Quantal is EOL.
The EOL announcement for Groovy was sent out today: http://lists.ros.org/pipermail/ros-re...