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From the Model Mayhem FAQ:

How long do subscriptions last, and how much does each level cost?
Both Premium and VIP subscriptions come in 1 month, 6 month, and 1 year recurring cycles. (The 6 month subscription is only available for subscribers who pay with a credit card.)

Premium costs $6 for one month, $35 for 6 months, and $60 for a year.

VIP costs $10 for one month, $55 for 6 months, and $100 for a year.

Please note that the cost-per-month is only lowered if a longer subscription time is purchased.