Arc3 Gases does an exchange, $19.80 for the 5# and $34.76 for the 20# cylinder. Math = $44.44 savings with the 20# cylinder. The #20 cylinder was $134 up front and came filled (of course). How long a cylinder last is highly dependent on pressure transfers, CO2 flushing, beer carbonate level, but should not include leaked volumes. I found disconnecting the kegs, pressurizing the lines and regulator and then turn off the cylinder valve and then check the pressure the next day. It's a yes or no approach, which then can be followed with soapy water if needed. Every time you change the cylinder, close the line out of the regulator and close the cylinder valve after pressurizing the regulator and wait overnight to confirm the new cylinder connection isn't leaking.
BTW I know this is a no-brainer but the cost should always include your time and mileage! When you brew long enough you'll rethink everything looking to save money as well as ways to improve your beer. If you're not already, start buying in bulk and/or online if the LHBS is too far.
Something else to consider.....be sure your CO2 is beverage grade. No one likes that cardboard taste. I personally have never had a problem with the CO2 quality just read about it. My cardboard days were back when I was still bottling my beer.