Can you spend the plot point on the very roll that will give it to you? (is that even a relevant question, wikipedia claims you can use cortex plot points for extra dice but I'm not familiar with the system, if they can only be used for non-roll benefits then it doesn't really apply.)Not wanting to do the thing to get a Plot Point is fairly obviously different than not being able to get a Plot Point.
There is a cost to getting a PP in Cortex Prime, yes, but they are effectively unlimited. You can always get one if you're willing to pay the (minimal) price.