Currently, ticket holders pay a penalty equal to 25% of the ticket price when they change their flight plans.

penalty 是名词，来自动词 penalize（处罚），指因违反规定或违约而受到的金钱惩罚或不利后果。在句中特指航空公司对乘客更改航班计划收取的额外费用，即"罚金"。