Caveat; I travel with an infant most of the time, so I never get upgrade offers via email. These are instead my experiences upgrading at the airport:
DOH-CPH: 2 pax + infant from H to J: 600+600+120 USD
DOH-CPH: 2 pax + infant from H to J: 600+600+120 USD
DOH-AMS: 2 pax + infant from H to J: 600+600+120 USD
AMS-DOH: 2 pax + infant from H to J: 525+525+0(?) Euro
Other than that, I have always been able to use Qmiles to upgrade all my flights; usually DOH-CPH or DOH-LHR.
10 J flights this year with QR - only 2 of them have been revenue tickets....