There's probably only 80 different commercial airplane models. This testing probably costs around $1000/hr - figure 5 days testing @ 8 hours a day = 3200 hours = 3.2 million dollars. Seems to be a reasonable price to pay.
EDIT: Almost forgot: After the testing is complete, you have to pay to have a test report written up and published, and then you have to pay for the FAA certification process. Probably not cheap.