The US authorities have just updated the FAQ section of the esta.cbp.dhs.gov portal and officially announced that, as of September 30, 2025, the fee for obtaining an Electronic System for Travel Authorization (ESTA) will increase from $21 to $40.
This is a significant increase that will affect all travelers using the U.S. Visa Waiver Program (VWP) to enter the United States.
I was noting in this comment before I figured I should post it, the ESTA is valid for 2 years so if you're looking to go any time in the next couple of years, applying now could save you some cash.

OzB post
https://www.ozbargain.com.au/node/923349