The honest answer is: it depends.
It depends on the per item manufacturing cost. And it depends on your landed cost. It also depends on your retail price. And it depends on how competitive your product niche is.
Some costs are easier to estimate than others. Landed costs are pretty easy to estimate once you have an idea of manufacturing and shipping costs.
But if you are serious about becoming an Amazon private label FBA seller then the best way to estimate your total upfront investment required to launch a product is by creating a 'dummy listing.'
Once you have your product category and size and weight you create a listing for it in Amazon Seller Central but then suppress it.
This will give you rough estimates of Amazon's referral and fulfillment fees.