I like it! What about this variation? Stone shows the number of pulls so far, but the number of pulls to win is randomized rather than being a fixed number. The higher the RNG required pull count for that particular prize, the better the prize. This would add a little more excitement and less potential for gaming the system.
Perhaps to keep the momentum going and incentivize players to buy pulls even right after someone just announced a win, there could be a secondary lottery aspect where each pull you buy (or first X number of pulls?) gets a lottery ticket to a lottery which is drawn concurrently when someone wins the main prize.