The conference fee will be announced after the summer of 2023.
It is hard to estimate the precise fee now due to the current global inflation, but HTC conferences traditionally keep relatively low conference fees.
As a reference, the regular conference fees in the past HTC conferences are as follows:
HTC 2015, Germany: 550 EUR
HTC 2022, Poland: 650 EUR
(HTC 2020 was held online and is excepted from the above list.)