It will vary depending on region, but you should have a look on a recruitment web page (reed, monster etc...) at similar jobs.
I'm a project manager (not by choice!), and get paid about �6k less than other project manager jobs in the area, but all PMs get the same here.
From experience, an assistant PM often gets paid a fair bit less due to the reduction in responsibility, and if a project goes wrong, it'll be the project manager who takes the blame (gets the sack!!), not the assistant manager.
Pay tends to be linked to responsiblity.