[austhrow743]

The idea behind napkin math is you start with something real or reasonable to assume as accurate and then extrapolate.

The original comment did exactly the same thing the person you're replying to did. They just pulled a vague feeling about the product out of their ass. That "3-4hr/week" saving is exactly as founded as this commenter's "$30 total".

That they wrote more and did math with their vague feeling is less logical not more. It's masking an emotional hunch with the sort of language people use when they actually are being logical.