...and you wonder: well someone must answer them otherwise they wouldn't bother sending them out? Well research has now been done!
It turns out that from 350 million spam emails sent out advertising pharmaceuticals, they'd have made 28 sales.
Kanich et al. (2008) worked this out by infiltrating a large botnet and altering the spam emails sent out so that they linked to a benign duplicate website under the researcher's control.
http://www.icsi.berkeley.edu/pubs/networking/2008-ccs-spamalytics.pdf