Too little plating (chromium?) on the tip? I bought a radio shack 8USD kit and the tip gets eaten away quickly if you leave it plugged in between securing parts on a PCB. The 30W power is too large for the tip. 20W would be probably better. Plus, mine is still fine but those ones I bought for students got their tips eaten up
I also have weller battery-powered irons. Their tips are pristine even after around 1,000 solder points. I suspect low power + good plating. Just too much hassle to replace battery every 200 points.