A major part of a successful FBA Amazon business is understanding the economics of both your product and the market you are entering on the Amazon marketplace.

The operational costs associated with selling on Amazon are:
Product cost
Shipping
Product prep (third party or self prep)
Shipping to Amazon
Amazon referral fee
Fulfillment fees including:
Order handling
Pick/pack
Weight handling
Amazon storage fee

Once you have your operational margin understood, think of your margin as your budget for Amazon Sponsored Products where you can reach millions of targeted buyers.