Ideally, Russia should pay for 100% of rebuilding Ukraine, which, last numbers I saw were circa $half a trillion, and that was before the dam collapsed.
It's getting pricey.
Realistically, getting Russia to pay even half seems unlikely.
An EU / NATO WWII Marshall-style plan should be roughly 2/3 EU and 1/3 US, but a lot depends on the politics and exactly how Russia's "special military operation" ends.