That seems a bit overkill. Couldn't he at least use two coconut halves for the U-235, a little bit of gunpower and a real fuse? Then at least you'd get a bang and a "clop!" when you set it off.
You've got two empty halves of coconuts and you're banging them together!
90 months divided by $550 = $6/month.
Wouldn't that actually be 0.1636 months/dollar?
10.0 times 0.1 is hardly ever 1.0.