I agree, the pricing/rental plans for propane and tanks is all over the place. Every company has its own pricing plans. Some offer inexpensive or free tank rental, but the price of the propane is higher. Others offer a higher fixed tank rental cost, but the propane is less expensive. Best to shop around and find one that fits your particular needs.
However, the difference between filling your car with gasoline, and having your propane tank filled is...with gasoline you go to them, with propane they come to you. There is a cost to delivering the product to your home...yes, it's often built-in to the price....but the frequency and volume of the delivery is definitely a factor in their operating costs.
So yes, if I was having a half tank of gasoline delivered to my house, I would expect the unit cost to be higher than having a full tank delivered.