Rules question: If you pay life to cast a spell, and the spell references your life total, is it your life total before you paid the life or after you paid the life?

It's whatever your life total is at the time. The important part is the order that the spell is cast in. There's generally only 2 times I can think of when your life total might be important:

If your life total is important in determining the total cost (like, "this spell costs X less to cast, where X is your life total), then it is whatever your life total is BEFORE you pay those costs.
If your life total is important in the spell's effect (like, double your life total), your life total will only matter when it goes to RESOLVE.

Fateful hour falls under your second example, not your first, correct?

That's correct! Break of Day, for example, doesn't care whether you are at 1 life of 50 when you cast it... it just cares what it is when it resolves.

EDIT:
Here's a more complex one; Faith's Shield. It might seem that it'd care when you cast it, but note that it has the replacement effect of "instead", meaning that you still target stuff with it, but the Fateful Hour part resolves INSTEAD of just the ordinary effect.