There are always trade offs when doing a repair. You can have speed but the cost goes up or the quality goes down. Keep the cost down and the time goes up or the quality falls.
This is what we do to keep costs down:
Discuss the fault directly with the customer - no middle person
Give an experienced estimate on the spot - if possible
Get an OK for the repair before we start.
Repair the thing with parts in stock or easily obtained.
Test it - make sure it is fixed.
Call the customer (you) to pick it up.
No wasted phone calls, no putting it back together several times to wait, no pondering what to fix first, no endless testing as each part is serviced. Very few revised estimates.
Occasionally, we will be incorrect. It happens. For us, it is cheaper to finish the repair than to take out staff time to revise a repair $20. The customer wins often.
Please keep in mind not everything is worth repairing. Costs can be too high.
Some items are worth repair but we cannot get the parts. We make parts often but this does raise the price of the repair. Give us a limit as we work.