Something that had no value when it was placed/discarded/dropped might be of considerable value now. Imagine if a discarded preparatory sketch by Leonardo Da Vince were to turn up in some Florentine cellar.
Wouldn't some grave goods fetch a decent return?
Definitely, and conversely many of them would have been more valuable when deposited than now. A polished pebble, for example. The reason for depositing grave goods was presumably that the objects did have value in the culture at the time.
It's interesting to speculate that there would have been fairly strict taboos associated with disturbing burials, so that the goods would be "safe", no matter how valuable. But it's also intersting to speculate that, despite such taboos, there might be unscrupulous types who would stoop to grave robbing. After all, why would the pharoahs' tombs have inscriptions warning of the fate likely to meet such an indivual, if no-one would have thought to act in that way. These days we have CCTV and ASBOs. Nevertheless, the sense of collective outrage when someone desecrates a modern grave or steals a bronze plaque from a war-memorial shows just how deep-seated the feeling is.