It all depends. I assume you are the viper owner who wants to rent the car to him. How much are you comfortable with charging people? As a business, price is reached using the amount of demand and competition in the marketplace. If you are just interested in letting a buddy "rent" it for a day call your insurance agent and tell them what you want to do with the car. That will give you the best indication on how to proceed. As far has how much, age, deposit, these are all things you should decide on using your own comfort level. I'm not comfortable renting to 21 year olds, they just don't have enough experience behind the wheel (Most of them). You may be O.K. with it if you find that your friend never "beats" on cars and is a very careful driver, etc. I don't know the customer normally, so I have to use "standards" for rental. I.E. 25 to rent, clean driving record, $ 1,500 deposit, etc. etc. It's just a lot easier to do it this way from a business point of view. Hope that helps.