According to a ministerial decision (No. 130022) taken in December 2020, there is a cap on the price for private COVID tests, both PCR and rapid.
The cap is set at €60 for a PCR test and €20 for a rapid test. The decision does specify that this cap does NOT apply for tests taken outside of private clinics.
You can read the text of the decision here (in Greek)
A reminder that tests offered by the public health service are without charge, if you are deemed to be in need of testing. If you have symptoms you should call the National Organisation for Public Health (EODY) 24H helpline on 1135 (available in Greek and English) and they will advise you.