Currently there is very little benefit for anyone to sign up for VIP once you achieve level 30. I think the majority of player base are at level 30.
If you want a more reliable revenue stream, consider revamping the VIP benefits. For example, for $4.99 a month (30 days), you get daily benefits of:
5 microchips
50 memorium
50 platinum
250K silver
2K gold
The daily rewards has to be attractive enough to attract users to login daily to claim the rewards.
I think this is a more stable income stream for the company, a more stable expense for the players knowing exactly what they will get, and it's a win/win!