Caitlyn Jenner -- Not Balling THAT Hard ... Her Lambo Is Just For Show

Caitlyn Jenner is not quite the lady pimp she appeared to be ... but she's definitely as showy.

As we told you ... a Lamborghini Aventador -- base price $400k -- showed up in her driveway on Tuesday ... via flatbed delivery truck. While we're sure she COULD afford one ... turns out Caitlyn did not buy it -- it's just a rental for her docuseries.

Sources close to "I Am Cait" production tell us the show rented the exotic whip for a scene they're shooting.

Remember, Caitlyn's still into Bruce's fave hobbies -- driving ATVs and motorcycles, so it's not shocking she might feel the need for Lambo speed.