You can rent a small car for as little as $30. Although their are many variables to consider like bigger or more luxurious cars will be more expensive. Also holiday, weekends and some days throughout the week might be more expensive. It is important to also note that some companies charge per miles driven while others charge a flat fee.
This question can't be answered unless I know what car you want to rent, how many miles between the cities, if you are going to buy the rental insurance, etc. I would suggest a midsize car and to check and see if your own personal insurance covers rental cars.
To rent a vehicle from one city to the next is usually around $30-$50 a day. It all depends on how far you are traveling and if you are returning the vehicle to the same company. Then they require you to make sure there is the same amount of gas in it as when you picked it up.
If you find a car that has unlimited mileage, with tax its about 60 dollars per day. Time of year drives the cost up. Also if it is a one way trip expect to pay a lot more.