The main factor in determining the price of a set of tefillin is the quality of the writing on the parchments.
The writing on tefillin parchments is considerably smaller than the writing on a mezuzah (in most cases) or a Sefer Torah. Small writing is hard to do well, so a set of tefillin parchments takes a lot of time and effort to write, making good tefillin parchments expensive.
Cheap tefillin are written more quickly. Often, they’ve been written so quickly that not only are the letters far from elegant, but they are barely kosher. Although there are G-d-fearing sofrim who write quickly, many fast writers are not 100% reliable. Even if the tefillin have been checked and certified, they may have just barely passed, relying on various leniencies, and are considered iffy.
Also, very inexpensive tefillin are written on parchment which has been treated to make it easier to write on, but this treatment accelerates the decay of the letters. So even if the letters do come out kosher, they’re going to decay in ten years.
Kosher tefillin peshutim
Kosher tefillin generally cost anywhere from $200 to $1,200 or more. (One online dealer sells tefillin for just $150. A friend of mine, who has been selling tefillin for over 20 years, says either they’re not kosher or they are selling them at a loss.)
If you are shopping at the low end of the price spectrum, you basically have a choice of buying inexpensive tefillin peshutim mehudarim for $200-$300 or tefillin dakkot for a bit more.
Many people will advise you to stick with decent tefillin gassot, which can last a lifetime if properly taken care of. I recently spoke with a tefillin craftsman who does repair work, and he told me there is very little he can do for tefillin dakkot that lose their shape. If you buy tefillin gassot, you will be getting better quality workmanship on the battim (boxes) and better quality writing on the klafim (parchments).
Quality tefillin: A small investment
Unless you only lay tefillin occasionally, tefillin gassot are definitely worth the investment. And although $500 or $600 may sound like a lot of money at first, isn’t your spiritual life worth a relatively small investment? Would you be willing to spend a similar amount on a sofa, a rug or a computer?
Take out a calculator and see how much a pair of tefillin will cost you over time. Let’s assume they last for 20 years. That’s about 6,000 tefillin-wearing days. If you spend $600 on a set of tefillin gassot, it’s costing you just $0.10 per day to connect to the Creator. That’s quite a deal if you ask me!
There are three covenants that link a Jew to the Almighty: Shabbos, bris milah and tefillin. Each of these is worth making a special effort, and you stand to gain immensely.