You may not like this answer, but I'll tell you anyway.
I used to work as the supervisor of PC and printer repairs for a company with approximately 150 offices spread over my state, servicing about 400 laser printers. I had four technicians working for me throughout the state and we were committed on-site service within 8 hours. The company paid for parts and I paid for labor and mileage. After about a year of operation, I found that printer problems were our Number One service call - by far - and most of them were for poor print quality which was resolved by simply changing the toner cartridge. The company was purchasing refilled toner to save the parts cost, but I was eating all that labor and mileage.
So I began a 2-year study collecting data to prove that these refilled toners were no good. It's been a few years and details are lost to time, but I collected somewhere in the neighborhood of 3000 incidents worth of data. I proved beyond a shadow of a doubt that refilled toner lasted, on average, only half as long as OEM toner, and that print quality was almost invariably worse almost from the beginning. Eventually it would deteriorate to the point the clients would no longer accept is and the toner would need changed before it was actually empty. I proved that even without the labor and mileage the refilled toners were more expensive because of their shorter life. I made a big presentation of all this data, but because of the way the contract was set up, the company was not willing to make the change. I changed my process and started leaving replacements with the office receptionists so they could make the change and most of our travelling service calls were no longer necessary.
So the short answer is: Buy the OEM toner and you will get full life and high quality throughout. As BattleGnome says, a modern printer should get at least 10,000 pages before needing a new toner (at the ISO standard of 5% coverage). For a $100 toner, that's just a penny per page. The paper costs four or five times as much - for cheap paper.
Now with the short answer out of the way, here's the WHY:
A typical toner cartridge contains a toner reservoir, a waste reservoir, and the imaging drum. Refill operations almost always empty the waste container and refill the toner reservoir (often re-using the actual toner) but do not replace the imaging drum. The imaging drum is an electrically- and light-reactive surface that works through static charge. To make an image, an electric charge is applied to the drum and the laser hits the surface to "draw" the image as a static charge. Then that portion of the drum is dragged through a toner bath and toner particles stick via static attraction. Then as the drum rolls around to contact the paper, the charge is negated so that the toner particles release from the drum and attach to the paper (which at this point is given a charge). Later in the paper path, a heated fuser will melt the toner and fix it to the paper. Then that portion of the imaging drum needs to be "reset" and cleared of the previous image so it's ready for the next bit of the image.
So the problems start when refill companies don't replace that imaging drum. The drums are engineered by printer companies to last as long as the toner particles last - with just a bit of extra time for redundancy. They're not intended to last for two or three refills of the toner, so they start to degrade. Maybe they don't take the charge as well and you get light printing. Maybe they don't clear charge the way they should and you get ghosting from the last revolution of the roller. Maybe they develop a general charge bias on the whole roller so you get an overall grayish cast to your prints. Maybe they get a knick or scratch that always holds a bit of toner so you get a repeating defect every time that spot rolls around (six or eight times per page) or even a line all the way down. You might even luck out and get a particular drum that's built better than standard and have good prints for an entire refill, but on average, that's not the case.
You can find toner companies advertised as "remanufactured" instead of just refilled which usually means that they replace the drum with a new one of their making. They are more expensive than simple refills, but cheaper than OEM. I still don't recommend these because the third-party drums do not perform as well as OEM ones; their quality is spotty and over time you'll probably just break-even compared to new. But if you're insistent on not buying OEM, definitely go this route. If you do, make sure you find an actual claim of replaced drums and don't rely on the "remanufactured" description alone.
With all that said, some brands separate the drum and toner into different maintenance items. In that case, a replacement toner cartridge is just a plastic box of toner and you should, by all means, use the cheap refills of toner but buy OEM drums. You can usually tell by seeing if toner cartridge and imaging drum are separate items on your printer's supplies parts list. Failing that, opening the flaps on the cartridge (carefully) will reveal the drum if it exists. They are usually a blueish, greenish, or grey roller with a translucent "depth" to them. Finally, you can just call the customer service number for your printer and ask them.