Christian Missions in Persia. Christianity was introduced in Persia in the Parthian period, and several bishoprics were established there (Latourette, II, pp. 263ff.).
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.
The average cost to refill a propane tank will depending upon the size of the tank, the geographical location, and the company doing the job. On average, a 10 pound tank is going to cost anywhere from $12 to $22 per tank. A 20-pound tank can cost upwards of $29 to $40 per tank to refill. A 100-pound tank can cost upwards of $65 to as much as $80 per tank. This is much cheaper per pound and will last a lot longer. Keep in mind that most stores will have a limit on the amount of propane that they can refill at any given time.