I thought this was very interesting about Brazil nuts and explains why they're so expensive...

Brazil nut trees produce fruit almost exclusively in pristine forests, as disturbed forests lack the large-body bees of the genera Bombus, Centris, Epicharis, Eulaema, and Xylocopa which are the only ones capable of pollinating the tree's flowers. Brazil nuts have been harvested from plantations but production is low and it is currently not economically viable. 
The Brazil nut tree's yellow flowers contain very sweet nectar and can only be pollinated by an insect strong enough to lift the coiled hood on the flower and with a tongue long enough to negotiate the complex coiled flower. For this reason, the Brazil nut's reproduction depends on the presence of the orchid Coryanthes vasquezii, which does not grow on the Brazil nut tree itself. The orchids produce a scent that attracts small male long-tongued orchid bees (Euglossa spp), as the male bees need that scent to attract females. The large female long-tongued orchid bee pollinates the Brazil nut tree. Without the orchid, the bees do not mate, and therefore the lack of bees means the fruit does not get pollinated.