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.
Propane is measured by the gallon so propane is priced per gallon. During the 2011-2012 heating season propane prices ranged from $2.74 to $2.88 per gallon, meaning that a 500-gallon tank cost between $1,100 and $1,150 to fill. Propane compared to other energy sources. That propane prices have increased over the last 10 years does not mean that, compared to other sources of energy it is necessarily more expensive.