I used to give them data, conversation was much easier, repeating my comment[1]
I take data from previous feature requests that were completed and number of bugs reported/refactoring etc. to create a rough cost estimate for maintenance in terms of man hours with the understanding that this is not perfect and there could be +-10% variation and this is only an estimate for planning. It worked well for me, other org's were not able to argue with me as i just showed them the data. Estimates were surprisingly accurate(its anecdotal) but i did not saw estimates overshooting/undershooting the +-10% variation. Later on this was adopted by the whole organization. I left but my guess is they are still using the same methodology
I take data from previous feature requests that were completed and number of bugs reported/refactoring etc. to create a rough cost estimate for maintenance in terms of man hours with the understanding that this is not perfect and there could be +-10% variation and this is only an estimate for planning. It worked well for me, other org's were not able to argue with me as i just showed them the data. Estimates were surprisingly accurate(its anecdotal) but i did not saw estimates overshooting/undershooting the +-10% variation. Later on this was adopted by the whole organization. I left but my guess is they are still using the same methodology
[1]https://news.ycombinator.com/item?id=31106593