PostgreSQL for Solaris is open source, integrated and included—for free!—with the Solaris 10, OpenSolaris, and Solaris Express Developer Edition packages. It is an install option for any of these Solaris distributions. Information on how to configure and run PostgreSQL for Solaris are included in the PostgreSQL for Solaris How-To Guide.
PostgreSQL for Solaris, like Solaris, is free to use as you please; you pay only when you want service and support.