The average size may give you a bit of a clue and in the end it...

The average size may give you a bit of a clue and in the end it is $/ct against cost to mine that matters.
Now this is all from memory so pardon if I am not exact!
Most mines avg diamond size is between 0.1-0.2cts and usually produce an avg price of around $100-$150/ct
Argyle was <0.1 and around $25/ct
Merlin is close to 0.3 with an price somewhere between $350-$500/ct
Ellendale E9 was around 0.4 with a price around $600/ct. E4 slightly smaller and $200/ct as not much fancy yellow.
The two samples quoted above came in at 0.7 and 0.6, now from small volumes you can't directly extrapolate but it is an indicator of the likely price direction. Other factors such as colour and quality then factor in also.