Our Gemara on Amud Beis discusses the principle of Hiddur Mitzvah. There is an additional requirement beyond fulfillment of a mitzvah; that is to honor a mitzvah by somehow beautifying it.

Rabbi Zeira said that for the embellishment of the performance of a mitzvah, one should spend up to one-third more than the cost.

There is an apparent dispute between Rashi and Tosafos regarding the interpretation of "⅓ more." According to Rashi, one should spend ⅓ more on the purchase price of the standard object to obtain a higher quality version of the same mitzvah object. Rashi provides examples such as purchasing a higher quality Sefer Torah, Tzitzis, Talis, and Lulav. In contrast, Tosafos suggests that one should increase the quality by a third more. For instance, if the minimum size of an esrog is that of a walnut, one should spend whatever it takes to acquire an esrog that is a third bigger. Presumably, Tosafos implies that whatever the standard quality is, one should invest the money to achieve a standard that is ⅓ better. This could apply to various situations, such as renovating a shul. According to Rashi, there is an obligation to spend ⅓ more to buy nicer materials, while according to Tosafos, one should spend whatever it takes to raise the quality by a third. Sometimes Rashi's approach could be more expensive, while at other times, Tosafos' approach might cost more.

The underlying basis for the dispute between Rashi and Tosafos is not entirely clear, but it can be understood in the context of the "what." According to Rashi, the mitzvah is to honor the mitzvah through action and personal sacrifice to beautify it. The obligation is essentially infinite, but the Torah limited it to a third extra in price for practicality, as it might be impractical to spend more than that. In contrast, according to Tosafos, the focus is on achieving a threshold of honor that is a third higher in quality. The limit of a third is not so much about practicality but more about fulfilling an enhancement. This distinction is critical because the third represents a financial limit for Rashi but is part of the definition of the enhancement itself for Tosafos.

I believe the dispute stems from how the proof text is understood.  The Gemara Shabbos 133b states:

דְּתַנְיָא: ״זֶה אֵלִי וְאַנְוֵהוּ״, הִתְנָאֵה לְפָנָיו בְּמִצְוֹת: עֲשֵׂה לְפָנָיו סוּכָּה נָאָה, וְלוּלָב נָאֶה, וְשׁוֹפָר נָאֶה, צִיצִית נָאָה, סֵפֶר תּוֹרָה נָאֶה, וְכָתוּב בּוֹ לִשְׁמוֹ בִּדְיוֹ נָאֶה, בְּקוּלְמוֹס נָאֶה, בְּלַבְלָר אוּמָּן, וְכוֹרְכוֹ בְּשִׁירָאִין נָאִין.

What is the source for the requirement of: “This is my God and I will glorify Him”? As it was taught in a baraisa with regard to the verse: “This is my God and I will glorify Him [anveihu], the Lord of my father and I will raise Him up.” The Sages interpreted anveihu homiletically as linguistically related to noi, beauty, and interpreted the verse: Beautify yourself before Him in mitzvot. Even if one fulfills the mitzva by performing it simply, it is nonetheless proper to perform the mitzva as beautifully as possible. Make before Him a beautiful sukka, a beautiful lulav, a beautiful shofar, beautiful ritual fringes, beautiful parchment for a Torah scroll, and write in it in His name in beautiful ink, with a beautiful quill by an expert scribe, and wrap the scroll in beautiful silk fabric.

Clearly the simple reading is in accordance with Tosafos, that the focus is to beautify the mitzvah.  There is no mention of cost (although practically it is set at ⅓).  However, let us examine Rashi’s explanation on the verse in Shemos (15:2):

וְאַנְוֵהוּ לְשׁוֹן נוֹי, אֲסַפֵּר נוֹיוֹ וְשִׁבְחוֹ לְבָאֵי עוֹלָם, כְּגוֹן "מַה דּוֹדֵךְ מִדּוֹד וְגוֹ' דּוֹדִי צַח וְאָדוֹם" (שיר השירים ה'), וְכָל הָעִנְיָן:

ואנוהו 

ואנוהו is that it has the sense of נוי, “beauty”, and the meaning is: I will relate His splendour and praiseworthiness to the inhabitants of the world.

Even more revealing is Rashi in Yoma (70a)

להראות חזותו לרבים - להראות נויו של ספר תורה ותפארת בעליה שטרח להתנאות במצוה שנאמר זה אלי ואנוהו התנאה לפניו במצות לולב נאה ספר תורה נאה בקלף נאה בדיו נאה בלבלר אומן ומערב יום הכפורים הביאום שם:

To show its beauty to the public - to display the splendor of the Torah scroll and the excellence of its owners, as one should strive to beautify the performance of a mitzvah, as it is written, 'This is my God, and I will glorify Him.' He adorned himself before Him with the mitzvah of the Lulav, [just as] a Torah scroll is beautiful with a fine script, beautiful ink, skilled penmanship, and on the eve of Yom Kippur, they brought it there

We see from Rashi that there is an emphasis on the effort placed by the owner of the object, and in seeing how much effort is put in, this shows honor for the mitzvah.  This helps us understand why Rashi sets the standard in a third more of the price, as it is about the sacrifice the owner is making as much as it is about beautifying the mitzvah itself. This also helps explain a Rashi in Pesachim (99b, “Lo Yochal”) that says it is a fulfillment of Hiddur Mitzvah to eat Matzah with appetite and gusto.  This is not about having tasty or crispy matzah, or certain chumros in the kashrus, although all that could also be Hiddur mitzvah. Instead, Rashi is focussing on the demonstration of attachment and personal devotion to the mitzvah, which can incidentally be fulfilled by beautifying the mitzvah but also in other forms, and principally in a form that demonstrates devotion and sacrifice. 

Rashi seems to hold that the mitzvah itself is less an object of veneration than the attitude toward the object.  It is almost as if to say that Mitzvos are only the medium, but the person’s attitude and mindset is the main concern. This is reminiscent of the following Midrash (Bereishis Rabah 44:1):

וְכִי מָה אִיכְפַּת לֵיהּ לְהַקָּדוֹשׁ בָּרוּךְ הוּא לְמִי שֶׁשּׁוֹחֵט מִן הַצַּוָּאר אוֹ מִי שֶׁשּׁוֹחֵט מִן הָעֹרֶף, הֱוֵי לֹא נִתְּנוּ הַמִּצְווֹת אֶלָּא לְצָרֵף בָּהֶם אֶת הַבְּרִיּוֹת.

Why should the Holy One blessed be He care whether one slaughters an animal from the throat or he slaughters it from the nape? Thus we learn that the mitzvot were given only to refine the creations with them.

Or, in other words, what is more important, the act of the mitzvah or the attitude and intentions GENERATED by the mitzvah? This might be related to how one interprets the question and responses pondered in Kiddushin (40b):   תַּלְמוּד גָּדוֹל אוֹ מַעֲשֶׂה גָּדוֹל Is study greater or is action greater?