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
"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.
Source: UK Gamespot