If you total rental or booking day less than 1day then the booking cost will be calculated as hourly price rate. You can set hourly price from product backend. It works as following.
Suppose you set per hour price as 10$ from product backend . Now if your total rental day becomes less than 1day like 2 hours then the booking cost will be 10*2 = 20 $.
If your total booking or rental days becomes more than 1 day then the hourly price will not work.