bwana.
your looking at it the wrong way round....kind of.
as you say buy the oppies at 3c today, and say in two weeks time the heads go to 50c and you want to convert your oppies to heads, the covert price is 20c for each share plus your purchase at 3c + 20c = 23cents so its realy only cost you 23cents all up but the heads are 50c.
hope that helps
