When the girls get their bills, out come the pocket calculators.
The main clause of your example has undergone subject-dependent inversion:
[the pocket calculators] come [out] -- (non-inverted)
[out] come [the pocket calculators] -- (subject-dependent inversion)
In version #1, the subject ("the pocket calculators") is located in its typical slot--before the verb. But in version #2, the subject has switched its location with that of the dependent "out".
There are reasons related to information packaging why a writer or speaker might prefer the inverted versions for certain contexts. Some of these reasons are: 1) newer info is often moved to the end of a clause, 2) info that is to be emphasized is often moved to the end of a clause for a stronger effect on the reader. Both of those reasons seem applicable to your example.
Notice how your original sentence reads better (imo) than this more basically structured version:
- The pocket calculators come out when the girls get their bills.
In your original version, "the pocket calculators"--by being placed at the end of the sentence--get emphasized to the reader. It was the pocket calculators that the writer wanted to be the main, and unexpected, point of the sentence.