In addition to costs varying, the number of birds tested varies by state. Some states only require a percentage of birds be tested. Missouri requires every bird over 4 months of age on the property be tested.
Some states may also require other tests besides P-T.
In Missouri, you can also have them tested for AI, MS, MG and Salmonella for an additional cost but those aren't required for NPIP certification.
If you only test for P-T, you may be limited to which states you can ship to since there are states that require source flocks at least be AI clean for importation.