I found kind of a suitable replacement. The energy drink I like the most is a very cheap lemon-soda energy with sweeteners instead of sugar (like 0,39€ per 330ml can). And instead of buying that, I went for basically the same, ie. lemon soda with zero sugar, just without the caffeine. It's not a 1:1 replacement, sadly - I'm sure I'd like the taste of the energy more - but it's close enough and only slightly more expensive at 0,59€ or 0,69€ for the same size