The Ray Sphere was a device used by transferring the neuro-electric energy from a group of people to concentrate it into a human indevidual to unlock his/her powers. Though it will unlock the dormant abilities of the Conduit, it will kill the catalysts, and worse, cause a catastrophic explosion.
How many gallons of gas does the average mid-size car hold? The average fuel tank size for mid-size cars is 16 gallons. If the  price of gas was $3.59 per gallon, it would cost $57.44 to fill a  16 gallon tank.
If you exchange, you are giving the propane company 5 pounds of propane and paying for the complete exchange. You will likely pay more for the exchange, even if your tank is completely empty. An empty 20 lb tank will hold about 4.7 gallons of propane. That is $18.47 at $3.93 per gallon, which is the low quantity price (I just paid $3.44 per gallon when filling several tanks). The exchange is about the same price. Keep in mind this comparison is as if the exchange gives you a FULL tank.