Most people eat an 8th and that could be anywhere from 40-60$. Eating any less then an 8th isn't worth it IMO. You feel like your on the verge of tripping but never do.
Anything less than an eighth is a waste, honestly. Lets say he gets some really shitty shrooms. 2g might not be enough to get him where he wants to be, but an eighth might. On the other hand, lets say he gets some super boomers. An eighth isn't enough for the experience to be so intense that he can't handle it, if he's already mentally sound. If he isn't mentally sound he shouldn't even be entertaining the idea of shrooming.