Note that this is referring to a skilled workman’s gross wage, not how much he could save in a year. Papyrus (from which we get our word paper) was the cheapest writing material available for a document of any length. To this expense, one must factor in not just ink but the wages of the educated professional required to meticulously copy each scroll by hand.

Another factor is that scrolls were bulky and each Gospel was kept to a size that was convenient to handle.

* This quote comes from: Morris, Leon The First Epistle of Paul to the Corinthians: An Introduction and Commentary The Tyndale Press, London, Revised Edition (1985), page 175