CPT® code 87999 phenotype tropism testing

CPT® 87999 is an unlisted microbiology code. The test is a phenotype tropism test.
This is prior authorized for payment and the billing is a high dollar amount. How should I pay for this procedure?
1. Is there an allowable amount that Medicare will pay?
2. What is the % or dollar amount I should use?
3.How do I find Medicares allowable for unlisted codes?

thankyou

Karen