This post gives a more detailed break down of the project manager and up positions (commercial archaeology UK) in the Jobs in British Archaeology series.
You can see the methodology here on how these numbers were collected from job postings. In total 14 job postings are represented here.
Lowest pay offered–
Highest pay offered–
Here is a distribution of starting pay (lowest pay) to give you a better idea of what pay is like for this position-
To figure out what the uppermost pay might be the difference, on average, between individual low pay, as shown above, and highest pay offered was 21.35%. So if you have several years of PM experience you can assume you will be making 10-20% more then the lowest pay offered. Though looking at the distribution above one could get the lowest pay offered and still make more then the highest pay offered at a different company. However, the range of difference goes from 6% to 41% so really it is best to weigh all of the options available.