I hit the 20k cap in about 5 mins selling Allagan pieces from my retainer, plus some random low level materia I had. In fact, I could cap far more than 20k every single week purely from the retainers Allagan pieces, which is zero effort on my part.
As for your point about reward, it doesn't matter if it's 3000 Gil, or 300,000 Gil, I am incentivised to sell my items at the enclave because they pay a higher amount. In fact, increasing the gratuity would increase the reward, whereas your idea of decreasing it would decrease the reward.