I had most of those cards when I actively played the game.
My highest rating was 2823, which is Diamond III, if I remember correctly, so they are fairly good. Nowadays I still play Splinterlands, but not so actively as before. Nowadays I mostly play other games.
Diamond is quite good. Didn't you get a lot of Dec from that??
I am honestly wanted to get into it again, so I get those sweet sweet Dec and maybe convert it into Cub ;)
I do not remember the exact values, but I earned back my initial investment ($10 USD) (Beta Starter Set), and I also made some profit. I mean profit much more than my initial investment, so it was indeed good. Nowadays the total value of my card collection is $50.63 USD, and I currently have only a small fraction of cards of what I had before.
You can always rent out your cards now, so you can earn passive Dec