Upvote:2
As pointed in a comment Switzerland is an extremely expensive country. Unless you actually work there, anything you want to buy or rent will come at an outrageous price.
So have a look at companies located in France. Mind that you may have to consider companies a bit further away from the border, anything close to the Swiss border is usually more expensive. There are trains and shuttle buses (cheapest option I believe) leaving from Geneva airport to different cities an hour or so away. Check if any rental company propose a pick up point in one of those cities.
Mind that if you plan to cross borders with this vehicle you might need to get this approved by the car rental company (insurance reasons).
Good luck :)