The meaning is "let's have a very special celebration". Even if there is no champagne involved, "allez, champagne!" means that we are not doing it the usual way, this time, we treat ourselves with only the best. For a music festival, they can say it, for instance, to emphasize the fact that they have a very prestigious guest this year.
But, depending on the context, it can also just mean "let's celebrate".
C'est notre 30ème anniversaire. Allez, champagne! >>> let's celebrate
Cette année pour notre 30ème anniversaire nous recevons XXXX. Allez, champagne! >>> nothing is too good for us, see how we treat ourselves.