If I buy Hub Protect now, but later want to add the combo Hub Protect + Remote Admin, does the Hub Protect value get prorated and applied to the combo subscription?
Yes. You can upgrade to the bundle at any time by using the "Upgrade" button.
There are several ways to find the button - my favorite is on the Subscriptions page (coming soon with 2.5.2 update - currently in beta).
Thanks. How would the current Hub Protect subscription value be applied in this case, per my original question?
Do you mind adding it to the cart (no need to check out) then share a screenshot for @holocron to see the calculation?
Well, that's embarrassing. There should be a proration for the service you already paid. We are looking into it.
I mean, you could have just said that way up there. 
I wanted to fully answer your question regarding the prorated value. I didn't want you to just take my word for it, that there will be one
Fair, but I think we all have a certain amount of trust here. Maybe not.
Certainly would be good to have the cart on the side show that and perhaps a note about it in the purchase flow. I haven't seen it so maybe it is there already.
That’s exactly how it should work. When a subscriber upgrades, the checkout page should display a prorated credit for the unused portion of the current subscription, the amount due that day, and the new annual renewal amount.
For example, if @chrisbvt recently subscribed to Hub Protect, the checkout screen should look similar to this mockup:
Could you please try again (may need to hard refresh your browser)?
I used Edge since I had not been on that site with Edge before, and I still get the same thing, without a price breakdown. Same exact screen as before.