Grammar question tags

[DanielG]

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?

[Piscean]

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.