In the rules for counting victory points it says every defeated or unit that fled off the board counts as points equal to the amount it costs to include in the army.
This leads me to believe that summoned units do not count toward victory points because they were not included in the cost of the army.
So do summoned units count for victory points?
No, they do not.
As you said, they did not cost anything to inlude in the army so are worth zero (unless the unit has a rule that states otherwise).
In addition to which, many summoning spells explicitly state this.
For example, Raise Dead:
Units summoned by this spell cannot be dispelled, and do not award victory points under any circumstances. (Vampire Counts 8th Edition, pg 60.)
