There are two basic tips to consider when looking for a cheap one way car rental... (if not cheap, at least less expensive!)
1. Keep the vehicle as few days as possible (1 day, if it all possible) for the one way portion of the rental.
2. Rent the smallest vehicle that will suit your needs at the cheapest location you can.
Basically, if you can pick up the car in Minneapolis and drop it off in Madison within 24 hours, and then rent a separate car in Madison for the other two days, this should help save you some money -- if the rental car company is charging a higher daily fee and not just a flat drop fee.
You can also try a full, 3 day one way rental, as well as the two one way rental cars scenario, at an off airport location instead, to see if that will save you some money. Airport car rental locations are generally quite a bit more expensive than local neighborhod locations, because of all the extra fees and surcharges.
This was $170 less than the cheapest airport pickup rate for the three day one way.
You can save even more money if you split the rental into 2 seperate rentals and rent off airport.
Again, through Hertz, I found that if you rented off airport in Minneapolis and dropped it off in Madison within 24 hours and then rented a seperate rental within Madison for two days, you could get the whole 3 days for under $250.
I hope these examples will help you.
While it's difficult to find a "cheap one way car rental", as you can see, you can save some money on your rental by either doing the full 3 day one way rental at an off airport location, or by splitting your trip into 2 separate rentals and picking up and dropping off at non airport locations..