In the late game the credit option becomes less appealing. Spend $90 a month and get 20 credits.
$600M is about 10-15 days in game revenue for whales not very appealing
I would suggest that the $90 a month comes with a 15% city, project, infra and land cost reduction that stacks with other effects.
I feel like this would boost the chances of larger whales purchasing credits and Between the $150M off of a city and the $600M + the build savings it may entice more whales without being game breaking.