It really depends on the number of chests in Silver vs the number of chests in Diamond, but chest for chest, I agree.
If you figure half as many chests per league you go up...
100 chests in Silver would give you an EV of $2.70
25 chests in Diamond would give you an EV of $1.16
There's also other factors to consider though. You could pull a lot more chests in Diamond than the example I gave, you have a higher chance at pulling packs in Diamond, the SPS multiplier is higher in Diamond, you have better odds of pulling GF cards, etc.
Lots to consider.
EDIT: Maths
Is this change related to chest rewards?
I think this change will only impact to the ranked battle SPS reward, and nothing related to chest rewards.
It won't affect what's in your chests but it will definitely affect earning them. If you're taking a huge penalty to your RP, it's going to take more battles for each chest you earn.
Good catch on individual chest EVs though. I'm not sure why I was originally calculating those with a penalty.