I have two at 4x4TB, so it's still much too expensive to go SSD. I was planning an SSD cache with 2x512GB SSDs in RAID1 for the next build but now I'm wondering

I have two at 4x4TB, so it's still much too expensive to go SSD. I was planning an SSD cache with 2x512GB SSDs in RAID1 for the next build but now I'm wondering how long it will take to be able to go all SSD.