This is a fun question for this month! I'm sure our members have lots of good answers. If I can think of one for me, it would be the variegated form of Agave attenuata that I got for $25 at Home Depot. This is a true variegated form with random yellow streaks on the leaves, not one of the striped cultivars. I've seen similar plants sold for hundreds of dollars. I just imagine the poor guy at the nursery when his boss found out they shipped this plant along with all the plain ones to Home Depot. What's your story of a rare or unusual plant in your garden?
I once had that variegated Agave attenuata (a striped cultivar, though) in my garden as well. Much as I liked it, I won't replace it because it is very vulnerable to frost. We had a very bad frost in the San Fernando Valley, several days in a row back in the early 2000s. All the leaves of that plant were killed back to the crown, but wanting that plant so much, I kept it to see if it would come back. It did, but without any variegated leaves. It lived out its brief, unvariegated, weak, discouraged life and expired a few years later.
I really like growing the uncommon but not really "rare" sea onion, Urginea maritima, and I have 2 distinct varieties: one acquired from a catalog, the other purchased at SoCalHort meeting from Joan Citron. They are named the same, but there are differences between them. My catalog plant has narrower, more wavy-edged leaves, and Joan's blooms sooner. I grow them next to paving in full sun.