Why is alcohol-free beer sometimes more expensive than regular beer?

It genuinely costs more to make, in most cases - it is not simply a premium price on a cheaper product.

Most alcohol-free beer starts life as a full-strength beer that is brewed as normal and then has the alcohol removed afterwards, using specialist equipment like vacuum distillation or reverse osmosis rigs. That is an extra production step - and extra equipment, energy and time - on top of a completely standard brew, not a shortcut. Smaller alcohol-free producers also do not get the same economies of scale that huge mainstream breweries enjoy.

On top of that, alcohol duty in the UK does not apply to drinks under 1.2% ABV - so alcohol-free beer is not more expensive because of tax, it is more expensive despite paying less tax. The extra production complexity outweighs the tax saving for a lot of brands, which is part of why prices have not fallen as fast as some drinkers expect.