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