As far as I am aware you can't update the Volvo SatNav to include speed cameras! The reason being that your system's data is on DVDs that are run from a player in the boot of the vehicle, (earlier models used CDs). The data on them is fixed so needs to be updated with new DVDs either each year or when Volvo bring out new ones - which as you have discovered cost a fortune and I don't think that Volvo are in the practice of showing safety cameras!
Ordinary SatNavs (TomTom/Garmin Etc) can be updated by the use of POIs (Points of Interest) via websites and downloaded from your computer to the SatNav, and some can be updated onto data storage areas within the SatNav whilst you are navigating.
DougD
November 2008