I use Brother printers, which allow refills. In fact, almost all printers allow refills, but you have to reset the cartridge in addition to refilling it. This is important because of the page count, like you mentioned - a printer says it is low on toner and some will reduce the page quality to get you to buy more toner even when there is plenty left in the cartridge. Shaking the cartridge doesn't cure this. Some have a spring on the toner cartridge that you reset, some have a series of buttons you press on the printer, and some have a chip on the toner cartridge that you replace.

Try Googling "reset" or "refill" and your specific printer model. Chances are there will be a YouTube video that helps.

I use two printers - my color printer (Brother HL-3070-CW) is worth refilling. My B&W printer (Brother DCP 7065DN) isn't worth it because I can buy off-brand cartridges for about $10 (I figure that's better than buying the powder and cleaning it up after the mess I always cause, but others may see a $6 bottle of toner as worthwhile).