I get 500 for each task from The Thieves Guild. I pick up one from Vex and one from Devlin then jsut fast track out and do them both and fast track back. Not even 5 min time. So thats 1000 gold. And there seems to be an infinite rotating quest cycle of those. And i have only done 1 quest for them so far.

Don't remember where i got it but i got 1500 from some quest that was repeatable. Think it might have been The Dark Brotherhood after finishing everything. Hail Sithis!