Q: I called in the local plumber to fix a burst pipe, but didn't confirm the price before he began working. How much do I owe him for the job?
A: When a price hasn't been set, the general rule is that we follow the accepted going rate, k'minhag hamedina. (Choshen Mishpat 331:2)
If there is a broadly accepted standard rate, this is easy. However, often there is a range of fees, with some workers charging more and some charging less. In this case, Ketzos Hachoshen (331:3) cites the Ritva that the workman, who is claiming payment, has the weaker hand, and can only demand the lower end of the price range. However, Pischei Choshen (IV:8(11)) suggests that if this particular workman has a known fee-scale for his jobs, or if his quality of work is of a particularly high standard, we would follow that pay scale.
To avoid misunderstandings and possibly cheating the worker, the Chofetz Chaim (end of Ahavas Chesed, Part I) strongly recommends getting a price-quote before beginning any work.