There could be a number of reason why you are getting an error message.
-- may be the generic cartridge is not properly "aligned" inside the printer so it can not properly read the information as to the cartride status.
-- Does the printer give you an error message but can still print? IF yes, then you may have to put up with the message and ignore it as an slight annoyance.
-- try removing the offending cartridge and replace it with a new one. It might be that ONE particular cartridge which is causing all the problem.
-- try the maintenace program. Clean the head, calibrate the printer, do a nozzle check. Go to Control Panel and select "Printers and faxes" and select printer properties.
The reality is - You may not have a choice. This is the trade off between generic brand and genuine brand. For my previous printers - I have used generic brand and ink refill in and I foud that they are NOT worth it. The quality of the replacement ink is poor. Other people seem to do quite ok though and never seem to have any problems. You might find that you replace the cartridge more often so in reality, it is costing you more in the long run.
R210 uses 6 cartridges so you should only have to replace a cartridge that has run out, so this should spread the cost out a bit rather than $20 X 6 in one hit. ($20 each??? You live in Australia by any chance?) One way of cutting the cost is to only print in black if you don't really need it in colour. When I need to print out my emails from the web, I only use black as colour is not necessary. You can select this in the print preference under "advance" and select "grey scale". This helps reduce the cost too.
Also, as the answerer above said, keep the cartridge and print until you REALLY can't print anymore before replacing it. I have done this too. You might be surprise how much more ink is left in there before print quality is affected.
Hope this helps