The most significant factor is that the mezuzah is hand written by a trained scribe. That is, 713 letters on 22 lines that are formed by hand! Unlike a mezuzah case that in some cases can be mass produced, each mezuzah scroll is created one letter at a time.
Next, in order to be sure that the mezuzah scroll is valid in needs to be checked and is usually checked three times.
- Once by the scribe who wrote it: Since while writing it's possible that attention is focused on the writing which may lead to errors.
- Once by computer: Since a human mind that is used to reading the words may miss out on missing letters.
- Once by an independent scribe: So that there is an independent perspective, not just the scribe who is profiting from the sale.
Also, the Scroll itself is made on parchment and uses hand made ink.
Additionally, if there is an error in the scroll, in many cases it is not repairable which leads to the loss of the hours of work put in by the scribe.
All said, much care and effort is put into the making of a Kosher mezuzah scroll so that you, the consumer will have the proper Mezuzah as it was intended to be. When you consider the cost of an authentic Mezuzah scroll over it's average life span it's far less than the cost of a tissue a day.