I don't understand what you are asking here. But please let me explain:
Imagine that the cheapest serial plan that the optimizer comes up with hast the cost of 20.
20 is higher than 5, which is your cost threshold. So, it didn't find a serial plan which is cheaper than your threshold.
I.e., the optimizer now goes and try to find a parallell plan, and that is the one you posted.
Since you didn't post the serial plan, we can't possibly say what the cheapest serial plan is. If you force a serial plan and post the one you get, we can see what cost it has. It should be higher than 5.