The answer is , it depends on the processor you have. You can buy a processor for as low as $5. However, if you want to buy a really high-end processor, you can get it for as high as $400.
The price of processor is not only dependent on the processor but also on the price of the motherboard. If the motherboard is cheap, then the price of the processor will be cheap.
The price of processor is also dependent on the brand. If the processor is made by a well-known brand, then the price of the processor will be high.