Whatever you paid for it when you checked out is all you will have to pay. It's likely out of stock because you bought the last one.
Exactly. Whatever the price was when you bought it, it is now locked in.
I frequently see prices go up and down. One example is the collector's edition of Tales of Beedle the Bard. At one time, you could get it for $95. Now, it's over three times that price.
In fact, while checking on it, it went from $315 up to $327.88 between the time when I searched and when I clicked on the product listing info.
I have things saved in my cart, or in the wishlist. I see it happen all the time where there's a message saying the price on this has gone up or down since it was added.
But as long as you've locked in the price, which you have, you've nothing to worry about.