Answer:
$5 million
Step-by-step explanation:
The initial investment of $5 million is now a sunk cost and is essentially irrelevant to future decision making.
If you stop production you will have spent $5 million.
If you continue, you will still have spent $5 million.
However,
if you spend an additional $4 million you will make $2 million giving you a loss of $2 million.
If you want to look at this mathematically:
Cost of stopping production: $5 million
Cost of continuing:
= $5 million + $4 million - $2 million
= $7 million
As you can see, the cost of continuing is higher. So, he will be willing to pay the amount that is below $5 million because it costs him $7 million to get $2 million in sales.
Therefore, the most he should pay to complete development is $5 million that is the cost of stopping production.