Traditionally work on FDM printers is priced according to the volume of the model: $/cm3 model volume. To make that work, somebody has to add all costs over a longer period of time and divide it by the cubic centimeters of model produced in that time. 

A more accurate way is to use a combination of two cost factors: Printing time and material use. 

In DigiFabster you can do both: price per cubic centimeter, and price per printing time+material used. 

If you use the $/cm option,  the file will be (if necessary) be repaired and measured, and the model volume measurement in cm3 multiplied by the price per cm3, and that's it. Different print qualities (layer thicknesses) will not influence the price, since they don't change the model volume, only the way it is printed. 

If you use the printing time/material use option you can set up different printing qualities (layer thicknesses) and the price will change accordingly because the time to print the model will change. An extra bonus is that in the case you specify material use, it will include the supports. The implicit settings for the supports are: supports start at an overhang of 45 degrees, and supports have a 15% infill. 

How to use the printing time/material use option: 

Go to the Materials Tab, choose set up a new printer, choose FDM, select your printer or create a new one, and input a price per hour. 

For a relatively cheap printer, like an Ultimaker 2, 10$ per hour is more or less standard, for the much more expensive Stratasys models you might want up to 50$ per hour. 

Save this setup. Now add a material. 

Just choose the material and go to the execution page, leave everything else empty. 

On the execution page use the Add button under Layer thickness and create an additional layer thickness (quality).
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Click Save.
Now go to test upload, and upload a model.
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After uploading you will find that the same model with the same material has different costs for different print qualities:
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To set up material use so it takes into account the material used for supports, you have to use the density/price per gram pricing parameter. 

Go back to the material set up earlier, by clicking on the printer name:
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Then click on the material name:
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When the screen opens go to "Density" and "Price per gram", don't use price per cm3
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ABS has a density of roughly 1,05 and is sold at roughly 30$ per kilo.
So fill in: Density 1.05, price per gram 30/1000=0.03$. Again, if you're working with a Stratasys machine, material cost will be roughly 10 times higher. 

To test if the supports calculation works it is easiest to first put printing time cost to zero again. Go to the materials page and click the "edit printer" symbol:

Delete the 10$ per hour and save.

 Now load two regularly shaped models, so you can predict the volume of the supports yourself. I use two identical cubes, both 100x100x100, one flipped through 45 degrees, the other through 65 degrees:
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For the cube at 45 degrees, I get a price of 34.39. At a price of 0.03 per gram, that's 1146 grams, at a density of 1.05 that's 1091 cm3. The space taken up by the model is 1000 cm3, the space taken up by the supports at a 45 degrees overhang is 500 cubic centimeters. At 15 % infill that's 75 cm3, with 16 cm3 remaining for the build plate adhesion. 

For the cube at 65 degrees supports will only go under one side, so the support volume is much smaller, plus the supports are lower. This is reflected in the price: 32,87/0.03 is 1095 grams, at density 1.05 that's 1042 cm3. The space taken up by the model is again 1000 cm3, the space under the right side of the cube is (89*44*100)/2=195800mm3=196cm3. 15% of that is 29 cm3, with 13 cm3 remaining for build plate adhesion. 

Ok, that works. 

Now restore the price per hour for the printer, so click the symbol "edit printer"
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input 10$, and save.
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When I upload the two identical cubes again, I can combine Quality and angle of inclination to get a range of prices: From High Quality with the 45 degrees cube to Standard Quality for the 65 degrees cube:
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That is to say: from 411.89$ to 145.01$, all for the same cube. 

Had I chosen the traditional method of pricing per cm3, and averaged at (411,89+145.01)/2/1000=0.28$/cm3, they would all have cost 280$.