If you want to be a Machmer and be m’chabud/Honor the Holy Shabbos it would for sure be nice to have a special pair. If it looks not shabbosdig then it is probably not an honor. But your not a mechlul shabbos if thats all you can afford.
I knew a person from Pre war Eroupe who even war a different Yarmeluka and and made sure never to wear pure black pant on shabbos. So it should be sepecial for Shabbos.
That’s why many Rebbe’s wear White pants on shabbos. That you should see it’s different from a whole week.