I recently tried the Avast Free custom installation and didn't face any SafePrice issues. On the install/custom screen, there are two tiny text options at the bottom left. These options could be the reason for the prompt for SafePrice.
Avast's BehaviorShield is not at the same level of advancement, say, as Kaspersky's. For instance, when I downloaded a harmless sample with a download manager, Avast's FileShield detected it. However, when I repeated the test a few times, BehaviorShield began detecting the download manager as a trojan. I could replicate this behavior after reinstalling Avast.