The cost of the bike to you is the the price of the bike $300, plus whatever ancillary costs you incur in order to gain possession of the bike. The cost of producing the bike is a completely different number completely unaffected by the purchase price.
The prior example of him purchasing the bike for $1000, does not change the cost of the bike. The cost is set at $1000 plus ancillary costs. The cost of production is also set, the bike is made the money is sunk.
The cost will fluctuate depending on the acquirer. Say the price of a bike is $1000. If I steal it, but I buy a $1500 plane ticked to get to it, my cost is $1500 minimum. If I purchase it for $1000 and have no travel expenses my costs may only be $1000.
Cost isn't a simple function of supply and demand.
Price is more closely tied to supply and demand than cost.
But again, He's probably right and I'm probably wrong. Since I'm going to lose this one.