I had a pool put in and there was already an extisting hot tub that had a box in the middle of my backyard. When a tractor was digging the hole for the pool, it ripped the eletrical underground wiring.

Does anyone know about how much it costs to re wire and put in a new eletrical box?

Should cost you $0 since the contractor is responsible for the repair. Otherwise it can easily run $500+ but we don't know the details here like how long of a run to the panel and what type of wiring or cable was used

In addition to ballengers adive that the contractor that did the damage may need to cover costs,

See this link to a Sticky Note near the beginning of this forum:
https://www.askmehelpdesk.com/electrical-lighting/prices-electrical-work-345486.html

The excavation contractor may have a hold harmless clause in his contract for damage to unknown hidden conditions, unless you let him know there was ungerground wiring and it's location.

Even thou Code does not require this , warning tape should be installed above all underground wiring, 12 inch below grade.

Code currently only calls for warning tape above service entrance lateral conductors.