My grandfather apparently just kept around some used motor oil that he'd feed through the chainsaw. You can't go much cheaper than that!!! However, your calculation that a few gallons would pay for a new chain and bar as needed probably makes sense.
I do some pruning and firewood, but I don't think I've ever worn out a chain or a bar. I personally like new oil because it is cleaner. However, I don't get too picky. Perhaps I'll try to hunt down some of the Harvest King that did well in the Project Farm tests.