Hello there!
The best value for PartyPoints is only available when you've reached the Palladium Elite VIP level, but in general buying clearable bonuses is always better value than buying direct cash bonuses.
Here's a chart of the bonuses that need to be cleared, at what VIP level they become available, and what % cashback they are equivalent to.
Here's the same for direct cash bonuses, which I've already mentioned, will always be lower value per PartyPoint
Regards,
Richard