(Note the incorrect terminology in the second sentence of your original post, which may be confusing - unlike a noun, the plural form of a verb does NOT have an "s")
Firstly I would be loath to classify that as a collective noun - the subject clearly refers to a multitude of persons. Although I am not a native English speaker, the plural form (earn) just "feels right" to me.
That been said:
The way I understand it a collective noun (similar to the above) can still take a plural form if not functioning as a unit, eg:
The team of oxen are scattered over the field, vs:
the team of oxen pulls the cart.
One may therefore also argue that as not everyone earns the same amount, the group does not function as a unit. (I realise I am pushing it here).
