You don’t need 100 voice actors. A decent voice actor can do multiples voices. But it would still be expensive.

Do you really need 100 differents voices? Even AAA games with large budget probably have less voices, focusing on the most important characters.

Games often use tricks, showing text/subtiles instead of actual voices, and inserting random “hum” / “clear throat” / whistle sounds to make the interaction more lively, avoid complete silence.

I guess you’re not making a typical game and voice synthesis might make sense for your game. Hopefully you can specify somewhere you’re using AI for voice only, not for storyline nor other artwork, to reassure players.