Creating one block requires (on average) 2^48/65535*difficulty hashes, or 4295032833*difficulty, since difficulty 1 corresponds to a target of 0x00000000FFFF0000...

This means that with n MH/s, you need 4295.032833*difficulty/n seconds per block. Otherwise said, 50 BTC per (4295.032833*difficulty/n seconds), or n/(difficulty*85.90065666) BTC/s.

This equals:

- 0.6984812728*n/difficulty BTC/minute
- 41.90887637*n/difficulty BTC/hour
- 1005.813033*n/difficulty BTC/day
- 30000*n/difficulty BTC/month (approximate)
- 367000*n/difficulty BTC/year (approximate)