I do not understand what this "after-tax value" means. On the surface, it seems "good" because it is reducing the costs that are being assigned to me, but I would like to understand how/why it is being calculated in the first place.
Your ex should be claiming this on her taxes, because the lower income partner is supposed to make the claim. So unless her husband makes less than she does, it isn't him claiming it.
But anyways, here's what it looks like, with some made up numbers. There are caps, but since I'm making up the numbers I'm going to pretend they are under and not worry about it.
Say the daycare expenses are $2000 a year. Your ex claims that in her taxes, and it makes a difference of $300 for her refund. So the after-tax cost of daycare is only $1700. That's the part that gets shared between the parents.
Say the sports expenses are $700. Let's say that including this expense makes her refund $150 higher. So really, the sports only cost $550.
To get these numbers, people do a pretend tax return without the claims and then with them, and compare the results.
If she's not providing you with her tax returns so you can verify her claims and check the math to make sure she's done it right/honestly, then request them before reimbursing her anything. Also request copies of receipts for the expenses. Don't just blindly accept a letter that tells you it's down to 65% of the actual expense.