Let's say you want to spend a lot of money on tefillin. What are you paying for? There are various hiddurim ("enhancements") regarding the construction of the battim (casements). Sometimes people look high and low to spend as much money as possible on tefillin, and an unscrupulous dealer might sell him battim with a certain stringency which leading poskim have said is so inconsequential that a dealer cannot charge more for it.
Then there's the parchments. Either they're kosher or they're not, right? Well, yes and no.
Let's say instead of a complete set of tefillin, you want to buy parchments directly from the sofer (ritual scribe), then take them to a tefillin maker and have him insert them. One sofer wants $300, another is asking $400, another $500 and another $800. What's the difference?
There are hundreds of halachas regarding how the letters must be formed. In some cases, there may be certain questions about whether a certain letter is kosher. (To give a very basic example, a ר shouldn't look like a ד and a ד shouldn't look like a ר). And there can be various opinions as to how to rule on a certain question. A set of inexpensive parchments might rely on a leniency by one of the Rishonim or Achronim; on the other hand, a very expensive set of tefillin will meet the requirements of all of the approaches among the Rishonim and Achronim, even stringent opinions.
And there's another factor: the aesthetic quality of the writing. This is partly a matter of training and experience, but moreover, of inate talent. Some sofrim were simply born with a good hand, capable of producing very attractive writing (after extensive training, of course). This factor should not be equated with the halachic issues regarding the formation of the letters. The aesthetic caliber of the writing is not a matter of "more kosher," but more attractive.
Is there any reason to spend good money for more eye-pleasing writing? Actually yes:Zeh Keli v'anveihu, which means we are enjoined to perform mitzvahs using attractive mitzvah objects. For some people that means a very attractive mezuzah cover, for others a beautiful atara on their tallis or for others a fabulous set of tallis/tefillin bags. Or it could mean using a very nicely polished shofar or a finely crafted menorah or a challah cover with exquisite embroidery work.