The fact proves that the system can be quite cunning; genuine good deeds don't earn you rewards at all!
After failing once, Jiang Yuan gave up on the idea of negotiating with the Buddhist cultivators to donate again to exploit a loophole for rewards. He sat in a quiet room drinking tea, taking stock of his spoils along the way.
First and foremost is the most important - lifespan.
Firstly, the Soul-Devouring Demon Skill upgraded to Level Two, improving its efficiency; secondly, the realms of these hundred or so evil cultivators varied, with not a short lifespan among them. Some could even endure long enough for him to extract several decades!
When he first acquired the Soul-Devouring Demon Skill, the evil cultivators he encountered were probably quite weak, and they didn't have much lifespan. Each time, he could only purify a few months, and at most one or two years. The most a whole evil cultivator could contribute was just over a decade.
