The source for breaking a glass under the Chuppah according to many commentaries is a Tosfos in Berachos 31a. The Gemara there says the following- (taken from the Sefaria website)
רַב אָשֵׁי עֲבַד הִילּוּלָא לִבְרֵיהּ, חֲזַנְהוּ לְרַבָּנַן דַּהֲווֹ קָא בָדְחִי טוּבָא. אַיְּיתִי כָּסָא דְּזוּגִּיתָא חִיוָּרְתָּא, וְתַבַּר קַמַּיְהוּ, וְאִעֲצִיבוּ.
Rav Ashi made a wedding feast for his son and he saw the Sages, who were excessively joyous. He brought a cup of extremely valuable white glass and broke it before them, and they became sad.
Tosfos comments- From here, it became customary to break a cup at a wedding. This custom remains with us till today.
Seems to me that the whole point of it was to create a more serious atmosphere at the wedding, and that can only be accomplished by breaking an expensive glass/goblet etc, just like the white glass brought in the Gemara which was hard to find in the market (see Tosfos Shabbos 20b).
Why then do people use old and/or cheap glasses to break at the Chuppah, it seems to be negating the whole idea behind this custom?