As previosuly stated, the 'A' in A-GPS just stands for assisted. So long as you select settings which prevent the GPS applications from accessing mobile or data networks, you won't be using the assistance and should incurr no costs. The device will simply function in plain, unassisted GPS mode.
While you're in the settings, make sure you also set them to prevent the downloading of maps across the mobile network. If that starts happening for big chunks of mapping while the phone's roaming you could run up some serious data charges. Download all you need via broadband before travelling.