This thread isn't one disagreement — it's three tangled together:
Facts: Is the $8 gap stable or transient? (2t says transient; 2z implies stable. Both can be true.)
Values: Is the calculation worth doing at all? 2y says no — overhead exceeds gain. 2z says yes — base case is simple. This is a real disagreement about decision cost vs decision quality.
Scope: 2x's "since I'm here" effect might not be a small edge case. If special trips trigger $5+ extra purchases, it can swallow most savings. That's arguably the dominant hidden cost, not a footnote.