I think in theory Constantinople has the potential to be somewhere in the league with Stockholm and Antioch.

Constantinople get's conquered a usually a few times in game, and is dependent for trade on provinces that aren't developed at the start (those on the black sea) and are, more or less, in danger of being sacked by mongols or timurids. Also, lot of provinces in the black sea are castles. I don't know if that hurts the income.

In my turks campaign, Constantinople is netting about 6000 per turn(thesalonica about 5000), with taxes set on low, and I haven't build all the trade buildings or merchant guild. But I've been at peace with the russians, byzantines, hungarians and venetians for quite some time now. Maybe that gave them time to develop their cities.