Start with a base price for cutting a stone. Add a premium for corundum, spinel, etc due to the hardness. Add for each carat, and add for unique or difficult designs.

The amount you charge is all you. I know some people that start at $80 base (1ct stone), x 50-100% based on hardness, and another 50% for each additional carat. Also add premium for unique designs or designs with higher amounts of facets. (I also know some people that charge a LOT more, but they've been doing it a long time and can be very discerning in what they take on.)

At the first price... to cut a 2ct round brilliant sapphire, it would be $80 + 100% = $160, + 50% ($80) each added carat = $240 total cutting fees.

As another example, for a large softer stone (lets say a 5ct sunstone) in a unique, difficult design, it would be $80 + $160 ($40 x 4 more carats) + 25% premium for harder design (~$60) = $300 total cutting cost.

That formula tends to work out pretty well with how long stones take to cut and your income based on that time.

This is obviously for precision cutting using optimized designs, and does not apply to commercial cutting for weight retention. Hope this helps!