TOPIC: Cost Gain from execution

If a project makes a cost gain from execution, where does the gain go to?

The PMBOK (pg. 265), "...when opportunities are captured and resulting in cost savings, funds may be added to the contingency amount, or taken from the project as margin/profit."

Again, the PMBOK is not prescriptive (telling you what to do), but rather descriptive (what could happen). The other option is to simply run a budget surplus and depending upon how large the delta is this might be a fine way to go (if the savings is 5% or so). If it's a small project and the delta is large as a percentage of the project, you would probably add it as a catch-all contingency because running 40% under-budget is just not a meaningful way to run a project IMHO.

Hope this helps.