What is the % of mark up in a supply Chain?

Your question is kind of CONFIDENTIAL to every industry!

Different industries have different profit margin, but there are some hints.

Please find the following 'Product Cost to Retail Sales Price' RATIO of some industry that I know of for your reference:
(1) Home Appliance / personal care applicance (e.g. Refrigerator / hair-dryer) : 50% to 60%;
(2) Consumer electronics (e.g. Mobile Phone) : 20% to 30%;
(3) Cosmetics (e.g. brandname lipstick, eye-shadow, e.t.c.) : 5% to 10%
(4) Fashion (e.g. fashion with famous brandnames) : 10% to 20%

You can do some more research on the retail pricing of different brands in different stores before deciding your mark-up supplying price, which will become the retailers' Product Cost!

Hope it answers your question.