Two possibilities spring to mind.

Coca Cola is produced by local partners, 

http://www.coca-colacompany.com/our-company/bottler-web-sites 

it is possible that the local partner in Indonesia does not have the space on its production line to make diet coke, which means that it would have to be imported, hence the higher price.

The second thought that I had was to do with size. The screen shot you attached does not show the portion size, if diet coke is only available for example in 500ml, but the others are served as 330ml then that might explain the price difference.

Hope this helps

Why is Diet Coke more expensive than regular Coca-Cola and other soft drinks in Indonesia?
In terms of obligation in mitzvos, what is the difference between an Eved Knani and a woman? And if they are the same, why do we have two separate brachos for each in the morning prayers?
