I like the venue for the music festivals,what I do...

I like the venue for the music festivals,what I don't like about it is there is an unreasonable mark-up on wine purchased in the venue. $15.00 wine at the winery (just outside of the venue) is $25.00 at the wine station in the venue.The people that sell the wine are volunteers ,so why is there a $10.00 difference.I have been to a lot of wine festivals and the wine prices are always the same in the concert festivals or not.After you pay $34.70 for 2 on-line which is cheaper than at the gate you should not be over-priced for the wine.