How much is melanoma testing generally?

Depends on test. A general physical examination may the only thing needed, if no suspicious lesions are noted and the cost would be of a visit. If a suspicious lesion is noted and needs biopsy then the costs include the cost of surgical procedure and diagnostic work up by pathology.
Don't worry. Melanoma is so serious that any reasonable doctor will find a way for the testing costs to be paid for, if the patient is really unable to pay. Ask to talk with a social worker. A biopsy generally costs about $200 which includes the surgeon fee as well as the pathologist fee (who actually examines it under a microscope).