my van has a diesel engine.
My car has 115000 miles.
My car has an automatic transmission.
First, you didn't mention what part it is, so there is no way I can tell if that is reasonable or not. Many parts cost hundreds of dollars. As for what any part should cost is always a matter of supply and demand. The entire automotive parts industry is heavily controlled by it. Doesn't matter if its a complete motor or a small do dad under the hood. Either of them could be $10 to thousands of dollars. If no one wants any of the thousand motors a company has in a warehouse, they will need to get rid of them in anyway they can. If they have ten wiper blades and ten thousand people need them, they will be worth more than gold.
If you really want to know if that is a fair price, call around to local parts stores. Keep in mind, there will be many brands available and some are significantly better than others. Better quality parts take more time and resources to produce. They will cost more. So for an accurate cost comparison, you will need to compare the exact same part numbers.
Have a car question? Get free advice from our top-rated mechanics.
Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Save up to 30%