Historically, some Japanese voicebanks (like Hatsune Miku V4X ) had "Lite" versions for producers to test before buying. These were never officially free for commercial use, were harder to find, and often had restricted features (e.g., no vocal fry, lower render quality). Today, most companies have phased these out.
If you own an iPad or iPhone, "Vocaloid Neo" offers a free download. vocaloid voicebank free
The most common way to access official Vocaloid voicebanks for free is through limited-time trials offered by Yamaha or third-party developers: were harder to find