Nah… curare is more stylish
Interesting question as to what the best weight for a diamond bullet would be. Carat is a measure of mass, right? Or is it volume? Say the diamond was the same volume as an M-16 round, how much mass would a diamond of that size have? Is ten carats reasonable?
If so, with your price you coulf fill a 30 round banana clip for $1500.
So I grant that you could afford to fill a clip, but you couldn’t afford to use it as the standard ammunition of your Army of Darkness in its attempt to dominate the world. 
Curare? Mere curare? Everyone knows that the STYLISH assassin uses tetrodotoxin derived from puffer fish. And if your victim doesn’t die, he goes into a zombie-like coma, from which he can recover weeks later, only to track you down and kill you.
[homer]
“FUGU ME!”
[/homer]
OK, nobody can really come up with a use for a diamond bullet. How about a diamond rifle barrel? I understand that although diamond is very hard, it is also brittle. Wouldn’t a diamond rifle shatter pretty easily?