That seems too slow. It's more likely 3200 stones more per each next level, i.e.,
- Level 1: 800 (400 rubies),
- Level 2: 4000 (2000 rubies),
- Level 3: 7200 (3600 rubies),
- Level 4: 10400 (5200 rubies),
- Level 5: 13600 (6800 rubies),
- Level 6: 16800 (8400 rubies),
Can anyone confirm?
If the above is correct, then you need a total of 36000 stones or 18k rubies (buying the right specials, that's under $20... and it could go down to $15).
If you have some stones, see how much you're missing and divide by 2 to get the rubies. For example, if you have 16000 stones, you're missing 36000-16000 = 20000, which is 20000/2 = 10k rubies
Disclaimer: all of this is correct under the assumption that the above formula for the needed number of stones is also correct!