Gran Turismo 5 Prologue will apparently be a free download on the Playstation Network when it is released. According to Yamauchi in a Gamespot Hands on, the game will be released as a free experience on the PSN later this year. We contacted SCEI, and they said

Quote:
Yes, it will be free for download on the PSN, and we are thinking of charging---but doubt it. For now it is free.

Polyphony Digital's Gran Turismo 5 promises to possibly be the most exacting and detailed racing video game ever. The Gran Turismo 5 Prologue gives you an online-enabled sampler of the GT5 experience. In the Prologue, will be dozens of different cars, and a few variety of environments to explore before the official games grand debut.

New Source: Gamespot