How common is sebaceous gland carcinoma in a 28 yr/white/female. i am getting 'cyst' removed on lower lid & dr said it must be sent for pathology

Very rare: One report said that sebaceous gland carcinoma is less than 5% of all eyelid tumors, don't know age distribution or gender. Rare things sometimes happen, getting a specimen sent to pathology is the safest way to find out. It is a good practice.

Who is paying?: It is rare but dangerous. Mostly elderly women. If the lab fees will be a burden to you, not covered by insurance, ask the DR to request the lab not charge you. If it will be a hardship for you to pay the lab, ask the DR to consider that this is a very unlikely diagnosis in a 28 year old person.

Common enough: If I understand correctly, you're upset because your physician wants to send what you think is a simple benign cyst to pathology to know for sure. It's impossible to be certain on the gross, if you want to play a guessing game you are foolish, not sending it would be malpractice, and you'll be extremely sorry if a cancer is missed. Here's how a missed lesion looks before eating into the brain.