Why do tight tolerances increase machining cost?
Because precision changes the process, not just the number on the drawing. A tighter tolerance can require different machines, fixtures, setups, process control, temperature discipline, inspection and rework risk.
Is every tight tolerance expensive?
No. Cost depends on feature type, size, process, material, quantity, equipment and how the feature relates to other datums. The expensive mistake is applying precision where the function does not require it.
Why can one tolerance affect the whole routing?
If a critical feature cannot be held in the normal setup, the shop may need an extra operation, grinding, special fixturing or additional inspection. That cost repeats across the batch.
What should the drawing communicate?
Which relationships are functionally critical and which are not. If everything is treated as critical, the manufacturer has little room to choose an economical process.