The reason the prices drop is because these are CHEAP CHEAP quality lights, usually made in China.
The quality doesn't get worse...but more "performance shop" ebay sellers start picking these up and undercutting other sellers.
I've watched it happen for years with different model vehicles. A new style will come out and then these will just get lower and lower fast until they are just being auctioned off for whatever they can sell for, or sitting in someone's "ebay store."
Its the same as those Chinese copy pocketbikes. I bought one when they were first out on ebay for $450 + shipping. A month later I bought another one for $235. 2 months after that I bought a larger engine, full fairing model for $104. These weren't lucky steals...thats what the market turned out to be since so many people were buying and trying to undercut other sellers.
It happens fast with the lights too.