The price depends on several things. One thing for certain once they land in US all horses stay in USDA quarantine which is 2k. Mares and stallions have to go to additional quarantine which is very expensive. The flight itself can be anywhere between 5-7k. Depending on laws you might need pre export quarantine aswell.
My good mare cost me about 15k to come to Montreal from Ireland. But totally worth it best horse ever.
To fly to the US, horses need passports, vaccinations, and proper bloodwork drawn prior to arriving. Upon arriving at Quarantine horses are examined, fly sprayed, and blood work is drawn. Once quarantine doors are closed the horses must stay in the quarantine facility for 48 hours, with blood work drawn periodically. Upon confirmation that blood work is okay the horses can leave quarantine :)