Here is an interesting interview with someone who claims to be a former Nigerian 419 scammer, who was recently released
Note that this person's identity is unverified, so apply usual caveats to the material (and read some of the comments on pt2).
To condense some of the figures - roughly 1% of the people targetted responded to an advance fee scam. Of those, perhaps 1 in 20 then went on to actually send money.
These figures seem somewhat high, until you take into account some of "John"'s other comments, namely that these are for email addresses which are hand-harvested from particular online forums - doubtless where the vulnerable and somewhat gullible congregate
- rather than scattershot lists bought at wholesale.
Worth a read, and please remember to be careful out there.