Hmmm I thought everyone knew this. It's shows up in the tips all the time.
For common items, possible boost power is: 400, 440, 480.
For rares it's 1760, 1936 and 2112.
For epics I don't remember exactly but it's close to 3500, 4000, 4600.
Legacy mythicals are 15000-18000 or so.
It depends on item type and item color. Some of them are incorrectly categorized too and give the wrong amounts.