Clarification:
In the CSM codex, you can turn one of your own guys into a spawn or a daemon prince. In this case, they're still under your control, so you haven't really lost them. If such models are killed by your opponent later in the game, they are worth whatever victory points they were worth originally, before transformation.
In the Chaos Daemons codex, you can turn an opponent's model into a spawn (which is then under your control). When transformed, the model counts as slain, and immediately grants its Victory Points (per its point total on the army list). The resulting spawn under your control is worth no further Victory Points (as spawn do not have a points cost in the Daemon codex), but it is worth a Kill Point in any mission that keeps track of such things.
You're free to disagree, Keith, but I can't assign a VP total to a unit that doesn't have a points cost in that codex. Different codexes, different rules, different outcomes.