Student or Learner
In this question tag
The tickets cost us $50, didn't they?
Why do we say 'didn't' instead of 'don't' or 'aren't'
How does 'cost us' help to determine the correct answer?
The person uttering those words knew that they were speaking about a past-time situation.
If the tickets generally cost $50 when we buy then, then the tag is 'don't they'.
Context is essential with 'cost' - the past-tense form is the same as the present in all forms except the third person singular.