This only comes out to about 39000 a month which is a lot sure but that's over a year to get 500,000 rupees. A year at $20 a month comes out to be $240, making it tricky for a lot of people to afford. So most people only do diamond for a few months at a time which would come out at roughly 150k (at an estimate really) which really isn't that much if you consider a DC of glowstone is worth about 60k.
