Print quality for a toner cartridge is determined by the drum in the cartridge and the toner powder itself. For best quality, you want a new drum and genuine toner powder from the printer manufacturer itself, like HP etc.
Cheap toner is cheap because the cartridges are used and NOT from the original manufacturer of the printer. There is NO other way that it could be cheap. Pretty much the cheap cartridges fall into these two main categories:
1) Old drum & no toner ---> they fill it up with toner and leave the old drum. The print quality therefore can never be as good as a new cartridge but in most cases it is not much worse either!
2) Old drum & no toner ---> they replace the drum with a new one and fill it up with toner. The print quality is better than (1).
In both cases, the powder they use is their own and not from the printer manufacturer. The cheap ones on e-bay have fancy names like "refurbished" and "re-manufactured" but all of them fall into one of these two categories.
From my experience, the toner powder they use is pretty much just as good as the original stuff. The drum is the only thing that is of slightly lower quality but you'd be hard pressed to tell the difference between prints. The bigger concern is usually toner leaking due to shoddy workmanship. This is rare but does happen.
