It’s a risky investment. Not only is the volume sold very low, meaning you don’t earn very much over long periods of time, but there’s also a chance that the zephyr will be re-released. Most of the other freebie items, geckos, leopards etc. have been re-released, so it’s entirely possible the zephyr will also be re-released.
If you want to invest long term, invest in things that people paid CCP money for like the interbus shuttle, or for prizes given out during tournaments like the AT ships, the luxury yacht or the pax east pirate corvettes. Although the demand is low for most of these, at the very least it’s less probably that CCP will re-release them, meaning eventually given enough time you’ll make a profit from them.