In the 3-pack, each Artipia game that you get instead of a copy of Gear & Piston is $25. So, if you made two substitutions, you would add $50.
Example of substitutions for a 3-pack to the USA or to the EU countries:
Gear & Piston + Gear & Piston + Briefcase = $79 + $25 = $104
Gear & Piston + Briefcase + Drum Roll = $79 + $25 + $25 = $129
The same applies for the 6-pack, except the substitutions cost $20 each.