The citizenship procedure is paid for upon its conclusion. Some costs for submission of documents are paid while the procedure is ongoing.
Does the application appointment cost anything?
The costs of taking Austrian citizenship depend on whether the application is for a single person or for multiple family members, and also depends on the precise procedure that is used. The only cost of my application appointment was EUR 3.99 for a copy of the Niederschrift (transcript of the appointment), payable in cash or by card. The case officer does however have to notify the applicant about the envisaged costs of the procedure, which will be levied when citizenship is conferred.
Does it cost anything to renounce British Citizenship?
The current cost (since 6 April 2018) of renouncing British Citizenship is currently £372. A full list of charges for taking British citizenship or renouncing it is available here. The fee appears to go up annually at the start of the UK tax year on 6 April, and rose from £321 to £372.