First of all I think we need to understand the terminology Net Profit.
What you are looking for is probably Gross Profit report and not Net Profit.
Gross Profit is the Profit from Sales of Goods or Services without considering the Indirect Costs like Salary, Rent,etc.
Whereas Net Profit is Profit after deducting the indirect Expenses.
Let us take an example:
Let us say you sold Product A 50 Nos. at $100 each to Customer A.
Your Sales (Income) Amount is $5000.
The Cost of Goods Sold (Expense) for the above product is $60 each, therefore $3000 for the above Sale.
COGS is calculated by ERPNext based on the valuation of the Product, it may also include your Landed Costs if you post LCV after Purchase. So let us assume that $60 consists of $50 product valuation and $10 from Landed Costs.
Now your Gross Profit is Sales-Expense = $5000-$3000 = $2000 at the Invoice/Order level and $2000/5 each= $40 each at the item level.
You will get this Gross Profit at Invoice/Order, Item,Brand level from Gross Profit Report in ERPNext
Next for Net Profit, we have to consider the indirect costs, now attributing Indirect Costs to items is a very Subjective topic, therefore usually company view Net Profit at the Company level or at Cost Center level (Line of Business, Branch, Product Line or whatever you consider as a significant Cost or Profit Center).
If lets say you spend $2000 on Salary and $3000 on Rent that month, how will you attribute these on the Items or Invoices? Therefore it makes more sense to view Net Profit at a Company or Cost Center level rather than Item Level
I hope this makes sense.