Well honestly, only like...
6 shields (off the top of my head) actually get BETTER, and only marginally so.
They are:
Diamond Shield, +1 Defence
Jade shield, +1 Defence
Spine Carapace, +1 Defence
Hope, +1 Defence
Mirror Shield, +1 Defence
Permafrost Shield, +1 Defence
Out of those, none have a considerable increase in cost, (2 at most, Spine and Mirror don't even change) plus they account for less than half the shields in the game. (Remember, what you get is TOTALLY random).
Hope, Mirror Shield, and Jade shield Effects are pretty low defences anyways, so letting it use upgraded shield effects isn't actually that big of a buff.
On the other hand, compare with the other shield upgrades, where they don't actually CHANGE, but instead have a 1 or 2 quantum DECREASE in cost.
So, I think it's pretty much fair as it is.