Within all the lower castes, there are "grades", the better you perform, the higher your grade. The higher your grade, the better life you have, better accommodations, more personal belongings. Clan merchants may not be sitting on personal fortunes of millions or billions of C-Bills, but the Clan is supporting a lavish lifestyle (comparatively speaking, by Clan standards). So if a Clan business man cuts a good deal, that's the benefit, maybe you go up a grade. Maybe you get that promotion that takes up higher among the caste hierarchy. Maybe you get a nice prestigious job close to the Merchant Factor's office.
Within the Clans, you still have the small time business man, peddling goods, barely scraping by, under everyone else's heel. Then you have the successful executive with the office and his own personal ground car. You acquire power. Even in a capitalist society, some people would rather have power over wealth.