Films split roughly 50-60% of their ticket sales with theaters so the 402 million Dune made goes down dramatically just from that. Another major factor is marketing.
Large blockbuster tentpole films like Dune typically spend a gigantic amount on marketing their film to a global audience.
Dune’s budget was 165 million so a decent guess would be that they spent an additional 41-58 million on marketing but there is no definitive way to know how much marketing spend was used for a particular film. However, a rule of thumb that is often used is that typically a studio will spend about 25-35% of the production budget (separate) on marketing. So in Dunes case the production and marketing costs probably amounted to about 200-225 million total.