You didn't say which port. Assuming Port Canaveral,
DCL Know Before You Go under COVID Testing (click "read more") has the following:
Guests who do not upload pre-trip test results to the Safe Passage website by midnight prior to embarkation day will be tested at the terminal before boarding and will be charged for the cost of the test. The cost of embarkation tests for fully vaccinated Guests will be $74.00 (USD) plus tax for the initial antigen test, and, if needed, $125.00 (USD) plus tax for a confirmatory PCR test, charged at the time of service. For Canada sailings, the cost of embarkation tests for fully vaccinated Guests will be $95.00 (CAD) plus GST for the initial antigen test, and, if needed, $160.00 (CAD) plus GST for a confirmatory PCR test, charged at the time of service. Embarkation testing for Guests 4 years of age and under who are not vaccine-eligible because of age will be paid for by Disney Cruise Line.
Note that it could cost a vaccinated passenger $199 if you also need the confirmatory PCR test (if you test positive with antigen test, they give you a more sensitive PCR test to make sure).
https://disneycruise.disney.go.com/why-cruise-disney/experience-updates/us/