I disagree- it just reflects what the market value of a wordpress plugin is. Anyone who feels they're not fairly rewarded can just stop doing it. Nobody is forcing them to do the work.
Besides, they most likely have motivations other than money- there's no way anyone starts making wordpress plugins thinking they're going to get rich (at least I hope not!)
Maybe not rich but there are several plugin developers that make a decent amount of revenue by selling plugins. For example check out Dan Grossman and his review plugin: http://wpreviewsite.com
Besides, they most likely have motivations other than money- there's no way anyone starts making wordpress plugins thinking they're going to get rich (at least I hope not!)