Lowest price wine we found was:
Beringer White Zinfindel for $18.00 per bottle.....sells in the USA for $5.00 per bottle.
You can bring your own box wine on board. There are some very high qualilty box wines in the $20.00 range for a 3 liter box.
Remember your on a cruise ship.....they want to be profitable. If you buy a bottle on ship, and you don't drink it all, the wine steward will store the remainder for you at no charge.
Also remember if you have to ask the price, you probably can't afford it. Also look at the long range......spending $20.00 for a bottle of wine is immaterial over the long haul of life. Your on the cruise to enjoy life. Do it. Don't worry about an extra $20.00 to $40.00.