I am trying to generate a metric for a company cost savings goal. To do this, I want to estimate the savings we realized by using an open source web application rather than building it from scratch or buying a COTS solution. One step in the process is to estimate how much it would have cost us to develop the application ourselves. Unfortunately, I'm at a loss for a really simple way to do this without going through a full estimation process.
Since I have the source code, I would think there should be some heuristic that could give me a very rough estimate of developer hours needed to write it. Unfortunately, my web searches on the topic mostly turn up articles and opinions on how lines of code are not a good indicator of productivity or quality.
My best solution so far is to pick a number of lines a developer could write in a day and work out the number of developer hours from there. If I go with that method, I would like to have some (preferably research based) evidence to back up my claim of developer productivity.
The one thing I have going for me is that to generate my final metric, all I really need is a lower bound on the developer hours or cost of the project. The higher the estimate, the better my metric will be, but I would rather the estimation technique be unassailable than have a high number.
Is there a better way to estimate the value of an open-source project?