At this point, even the US government has to admit it has no solid evidence of Iranian government involvement, so the real question is why would the US government or a faction therein make up this bullshit in the first place?
If you look at the actions of the individuals concerned, the actual alleged plotters, their motive could conceivably be as plane suggests, to hike the price of oil for a short-term gain, but it's a hell of a gamble. Anyone betting serious money on a temporary hike would have to plow a lot of money into oil and gamble that the assassination (a) would go through and (b) would produce the necessary bump. I don't even know how he'd do it - - is there a futures market in spot oil? I'm sure there's a way, but the guy would have to have balls of steel to get in there and lay down his bet in advance.