Most travel pricing these days is based on supply and demand and not on actual cost. The exact same airplane seat, car, or hotel room can vary by a LOT depedning on what the provider thinks they can get. I've seen a the price for a regular Holiday Inn that typically costs around $100/night go up to over $1500 (that's a factor of over 15!)
There is not a whole lot a you can do about it other than "shopping around". Try different dates, different providers and or/different locations.
Unfortunately, there is no rhyme or reason for pricing, so "trial and error" is the way to go. Some things that I have observed
- Sometimes picking up a car in town is much cheaper than picking it up
in the airport.
- Local providers can be much cheaper than the big chains (observed in Mexico, Iceland). This might be tricky with one way, though.
- A 4 day rental can be substantially cheaper than a 3 day rental. You just have to make sure that there is no "early return penalty" that some companies actually do have (observed in Germany & Netherlands).
- Go through a third party provider (observed in Germany). Last time in Germany I went with check24.de which was less than half the price than booking the same car on Enterprise's website. Given the massive price difference I was worried about this, but it went great: Enterprise even had my frequent driver number on the booking and gave my a nice upgrade.
Just one more example on how crazy this all is: I had rented a car for about 10 days or so and needed to extend it by a single day. The provider quoted a price for this extra day that was extortionary high. Fortunately I was at some point close to the rental place so I just dropped it off a few days early and managed to get a refund for the remaining days. This refund more than paid for new rental with a different provider that included the extra day. So I ended up with a "free" extension, more money in my pocket and a nicer car.