What's the cost of x? The cost of x is the amount that you have to pay for goods or service. ( x). What's the cost ( of x) in Japanese yen? What's the cost ( of x) in wear and tear? We use 'in' to refer to 'how' you pay the price when you buy 'x'. It could be a currency, or it could be another cost, such as wear and tear on your equipment that you have to factor into your calculations. What's the cost (of x) to the customer? What's the cost ( of x) to the economy as a whole? We use 'to' to refer the person or other body that has to pay this amount or has to bear the losses.

What is the cost in euros/dollars/yen, etc.? What is the cost to me? (Although it's better to say, "How much/What will it cost me?" OR What is the cost to buy this on credit? What is the cost of this umbrella? (Although it's better to say, "How much does this umbrella cost?")