Unfortunately, ironically, that sounds like a clock that would take a long time to make.
A search on Google Play Store for "meeting cost clock" comes up with several, though the best looking from the screenshots appears to be the $1.99 Meeting Cents with almost no downloads and no updates since 2014. Several others are ad supported or low cost. Frankly it'd cost more in (billable?) time to test and select one than to buy all of them.
A very simple browser based implementation is at https://www.mcgurrin.info/clock.htm and another is at https://tobytripp.github.io/meeting-ticker/