First of all, there is no such product that gives you 100% profit as 100% profit is the uppermost limit which is impossible to achieve in reality. Every product comes with a cost and if there is a cost, it will eat out your profit.
Cost eats your profit and hence profit can't be 100%.
We are hardwired to include only Purchase Cost as 'Cost'. But in reality, it is just a slice of the huge pie.
Some of the invisible costs include
Inventory Cost/Holding Cost
Paid Cost vs Unpaid Cost(Credit)
Labor Cost
Utilities Cost(EB,Rent etc)
Marketing Cost
Logistic Cost
Taxes etc
Let us prove by comparing Pure Product Offer(1+1) and Hybrid of Cash(25%) and Product Offer(30%)
Let us buy Product A in these two Schemas
Pure Product Offer = No Cash Offer
Hybrid = Cash + Product offer(Best of both worlds)

Note:
In Pure Product Offer, when you buy 10 Qty you actually buy 20 Qty(1+1)
In Hybrid When you buy 10 Qty you actually buy 13 Qty(30% Product Offer) and 25% of cash in Hand at the disposal
Take a note of Profit per Qty(Last Column) - It is hardly a difference of 0.5 rupees.
This is just proof to show that 1+1 is just differed by a meagre amount in profit.
In this proof, I just considered Buying and Selling and tried to simplify the case as bottom as possible.
Next Post I will exactly prove HYBRID >> PURE PRODUCT OFFER. Stay tuned !!