why pay more when people who already have lines do trick?

That's basically the gist. Extra vs Co-star is really just the difference between line and no line, but operate at very different pay rates, and even are hired through different companies.

featured extras/ day-players have no lines but some focused camera time, still pretty cheap. they make more on the backend.

co-stars in the union could cost almost 1k for a day, whereas extras make about $100. If you're penny pinching, what would you avoid?

also, small fun fact, the dream is to be an extra plucked from obscurity and given a line to say, which starts a career or whatever.....that almost never happens. usually a co-star that auditioned to say that one line.

source: I've driven 40 minutes to say 1 line many times.