60%+ of cost on my bill in northern CA is delivery, not generation. Which is dumb, cause it's a fixed cost charged as variable, but that's a separate discussion.
The cost is almost entirely driven by peak use though, so what would actually be "fairer" (or at least more cost-reflective) is to charge the transmission cost based on your individual contribution to peak demand on the infrastructure. People kind of hate that though because it's difficult for most consumers to predict or control their contribution to peak demand, and for the same reason it doesn't drive behaviour change / more efficient use of the infrastructure as much as you'd like.
In the end it'd probably still be about 60% of your bill anyway.
They're both the exact same resource mix because it's the exact same grid.
Here's the resource mix https://feature.wecc.org/soti/topic-sections/resources/index...
The difference in cost is entirely contracting and last mile transmission.
https://www.wecc.org/_catalogs/masterpage/wecc/Images/WI-Map...
Also, just because 2 states are interconnected doesn't mean that the interconnects have enough capacity for electricity sources to be fungible.
Edit: After the 2003 blackouts the Eastern interconnect got much more... connected
https://upload.wikimedia.org/wikipedia/commons/d/d4/UnitedSt...