Is any approximate method to calculate a MHash/$ ?

You forgot one axis - TIME.

At the MH/s rate, it may take years to solve a block... then how many BTC will be in the block at that time?

The only thing that you know right now is the current exchange rate.

We don't know what the USD/BTC ratio will be tomorrow, but we know what it is now.

We can also figure out how long it SHOULD take to earn BTC at a certain hashrate, but that would be at the current difficulty of 4.8+M...

So there is a method to calculate how much one could 'earn' mining at a certain hashrate... but this is way easier.

Go to this website...

http://www.bitcoinx.com/profit/plug in the necessary numbers and voila!

You can guesstimate earning potential.

We all know that the difficulty will begin a parabolic rise... so play around with that - the exchange rate... all kinds of stuff.

Good luck!