As far as we can tell, the cheapest way to get into "Generation One" Transformers, a.k.a. the original Transformers series, is this Complete DVD Box Set with all 98 episodes plus the movie. That's Optimus Prime on 15 discs in an attractive tin case. Just a slight heads up, the discs are imported from China and therefore are primarily in Chinese. They do have an English audio track and we can easily vouch for it working.
You'll be the talk of the block, and you'll be able to remember how awesome it was relative to the Transformers Movie on DVD. Just saying. (thanks Vanessa).