You can interpret it in two ways:
- [number] + [unit] + [things that are measured]
[Eine] + [Milliarde] + [Euro] ist viel.
[Ein] + [Dutzend] + [Eier] liegt im Kühlschrank.
- [number] + [things that are counted]
[Eine Milliarde] + [Euro] sind viel.
[Ein Dutzend] + [Eier] liegen im Kühlschrank.
In interpretation 1 the word Milliarde is the name of a unit in which Euro is measured, and you have one unit of these things, and because it is 1 unit, the whole term is something in singular form, and if it's the subject of the sentence, then also verb must be used in singular form.
In interpretation 2 the word Milliarde is part of the number that tells you how many copies of "Euro" you have. And then the whole term is not 1 thing, but many things. This means it is something in plural form, and when it is the subject, then also the verb must be in plural.
Both interpretations are valid, and so both form are correct.