Really cheap, but do you really want horrible Sangria?
Ugh, that Polish-made stuff is intended for alkies. Cheap though. I'd go to Biedronka which has bearable Spanish and Bulgarian stuff for less than 15zl a bottle but the OP should get the best they can easily afford since, as you say, nasty wine will make nasty Sangria...
While I remember, some people swear that cheap wine can be made a bit better with a few second in a microwave, though I'm far from convinced.