The nice thing about the party is that everyone is in on the joke and they are in it together. They know from the beginning that there are going to be surprises and much of the conversation is around this. The host can avoid an advantage by putting the bottles into blank bags then getting someone else to apply the numbers.
Pour everyone a serving of each (different color cups works best), and tell them that one bottle costs less than $10, one costs $95.
The group will taste and discuss and usually reach a consensus on which wine is the better wine. Ask if anyone disagrees.
Then, you reveal that both bottles were filled with the same boxed wine. It's really funny.
I've done this 3 times, and never had anyone suggest that they were the same wine before the reveal.