I think people tend to underestimate just how artificial alcohol pricing is. Without taxes it would cost like soda (at least in theory).
That's similar to the cheapest wine, although such shops don't really sell the cheapest wine.
Some seasons of the year, home made wine can be found for even less.
The cheapest beer and the cheapest soda should be compared, since they both save significantly on marketing and brand costs.
In that case, soda is about 25¢ (ja! Apfelschorle) and beer 50¢ (5,0 original export). Both from rewe.de.