Treat magic ammunition as a consumable
Here is the description of consumable items on page 141 DMG:
Some items are used up when they are activated. A potion or an elixir must be swallowed, or an oil applied to the body. The writing vanishes from a scroll when it is read. Once used, a consumable item loses its magic.
Unfortunately, it is not that useful a definition, as it does not actually define what a consumable is. It only tells us some things that must be true once we have a consumable. It says that consumable items are used up when they are activated, and that a consumable item loses its magic one used. However, this is logically not the same as saying that all items that are used up or that lose their magice once used are consumables. Being used up is a neccesary, but not a sufficient condition. The defintion only tells us that if it is a consumable, it will be used up, not that if it used up it is a consumable.
Potions, oils and scrolls are given as examples. Potions and scrolls are also explicitly stated to be consumable items (p. 139), with oils mentioned under the header of potions. All instances of magic oils in the DMG are classified as potions on their type line, (p. 183f), so from a rules perspective, oils are just potions that are applied externally. But again, examples are just that -- the list does not tell us if these are all the types of consumables, or not.
When consumable magic items are mentioned elsewhere, the DMG uses formulations like "a consumable, such as a potion or a scroll (p. 129, 135), or "consumable items, such as potions or scrolls". The phrase "such as" implies that there should be other consumable items than just potions and scrolls.
Xanathar's Guide to Everything's optional rules for item creation likewise use language like "a consumable item like a potion or scroll" (p. 129, 133), and Tasha's Cauldron of Everything again uses the language "consumable item such as a potion or scroll". Again, this suggests there should be other consumables, or you could just say "a consumable (potion or scroll)".
However, no other item type besides potions or scrolls is labelled as a consumable.
Where does that leave us? In the narrowest reading, ammunition is never called a consumable, and we have no explicit rule to tell us what a consumable is, so while it awfully looks like it should be one, it would not be. While the remaining text also strongly suggests there should be other consumables, none are explicitly called out.
This seems very unsatisfying. For all practical purposes, magical ammunition is just as well consumed as potions or scrolls when you use it, and it seems wrong to have to pay full price for it (and honestly, even at half price, it is not really worth it).
Cost is the only point where the distinction actually matters, as the items already tell us if they lose their magic or vanish, and the rules to halve the cost are the only ones that cares about if a magic item is called a consumable or not.
I think a better approach to read the consumable defintion is to consider it as explaining by example what consumables are, and treat everything that behaves like a consumable, as a consumable.
The D&D Adventurer's League rules provides some support for this. The AL rules are not offical rules canon, but they are published by Wizards of the Coast. In the Adventurers league player's guide, the following definition is used instead:
Consumable items include any magic item that is consumed if used (potions, scrolls, ammunition, etc.).
While rather brief, this is a more useful definition. It still leaves something to be improved, because it is not entirely clear what this means for a Necklace of Fireballs or Robe of Useful Items, items that are partially consumed while used, but at least for ammunition, it provides clarity: you should treat it as a consumable.