I think the reason no one goes down there is that there's nothing worth hunting there. Now there is.
As a bonus, dimik's give decen exp, and Vesper is down there, so there's more incentive to go hunting than to just sit around buying.
Singing plants may've been easier to get due to the ease of finding metallers, but metallers are worthless for exp and the things around them are as well, so there's less incentive to do that. Additionally, it's 2.5x as likely to get a transparent plate from a dimik as it is to get a singing plant from a metaller.
Finally, you only need 1 of each transparent plate, but you need 100 singing plants, so the demand will be lower than the demand for singing plants.
As such, I don't believe that the prices of transparent plates should be more than 1M max, and will probably be as cheap as 750k...