I mean, I feel like the original Sunbow G1 cartoon was probably made pretty cheaply, all things considered. Sunbow was known for putting out shows fast and cheap. So, I would think that show would be close to being the cheapest series Transformers has ever made. The Movie might skew that. But, if we're just talking about the cartoon series itself, it might be within the rankings.
Even cheaper than that would be the Generation 2 cartoon. But, counting that feels like cheating a bit. That show was just re-airing old G1 episodes with a new intro and framing sequence. They spared every expense! But, it kind of feels like that breaks the spirit of the question, a bit.
tfwiki.net
Botbots might be down there. If only because the episodes are short and they don't have that many episodes.
But, probably the cheapest series that I can think of would be Q-Transformers. A series of Japanese flash shorts released mainly on Youtube and similar platforms.
tfwiki.net
I'm going to guess that Q-Transformers is the series that they spent the least amount of money on in the entire franchise.