One example could be: the 3rd party, cheap grip blows your £3k camera up, try getting an image with it then, regardless of what lens/UV filter or memory card you have for it.
You have to understand these chinese copy grips go through next to no QC and the manufacturers could not care one tiny amount about you or your camera.
As has been said, there is no right or wrong answer here because its down to what you perceive value in, personally I dont consider the price of an item to be the reason for buying, I look at the quality and overall reason behind the product.
10% of the value of the camera to spend on a grip is not excessive, this would fall in line with grips for 7D etc (circa £100-£120) but at the end of the day if you feel its too expensive then dont buy it, no one has a gun to your head. From my experience of using genuine, original kit (touch wood) I have never had an issue, in fact the ONLY time I have ever had an issue with anything was when I first started out I bought a Sigma 70-300 and it had to be repaired under warranty as the AF stopped working. Since then I have stuck with Canon kit and L lens and never had an issue. For me, its worth the extra but like I said, I dont consider the cost of something to be the reason for buying it, or not.