So the most recent patch that updated the experience of the daily quests and reduced the max cost of stamina buys from 400 to 200 also reduced the cost of the 4th buy from 100 to 50 diamonds.
This seems like a small change, but for those of us who aren't big spenders it's really nice. In fact, over the past couple months, they've doubled the amount of 50 diamond stamina buys. I'm assuming that they did research on how many people were buying each level of the stamina buys and that they determined the best route to take was to lower the cost of the buys, encouraging people to spend even more diamonds on a regular basis and then augment that with a plethora of stamina resets.
My daily routine has always been to buy the 50 diamond buys and that's it, unless there's a double drop event going in which case I'll do all the 100 buys as well. So I've now gone from 100 diamonds a day to 200 a day, meaning I save up less for my big expenditures. On the flip side, when I do use all my stam buys, I end up spending less because they've reduced the cost ... which means I have more diamonds to spend. So I need more stam resets ... which they have been throwing in every bundle recently!
It's the rare decision that seems to benefit both GREE and the players, and I thought that should be applauded.