My daughter was quoted $700 to fix horn.

Is it possible it could cost this much?

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Most certainly should not cost that much. But electrical is extremely uncertain to diagnose, may take 5-minutes or 5-hours.
So they give you worse-case-scenario. Also depends upon how much you want to do vs. how much you want them to do.

Some very simple tests:

1. Disconnect horn. Apply +12v from jumper-cables directly to horn. Does it beep?

2. Connect multimeter to horn connector-plug. Set to 0-20v range.
Have assistant push on horn button. Does it get +12v power at horn plug?

Those two very basic tests will give you tons of info on what may be problem and how to fix it.
Heck, could be as easy as fuse (5-minute fix).

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