How does average part cost work?

it seems I can’t get a properly consistent number when putting in unit cost for parts. for example I put the unit cost as lets say 100 for 5 parts. I had zero parts at the moment so its not two averages adding up. the average i get is somewhere around 99. I try to revise it to 20 for unit cost thinking I got it wrong, it happens but now the average cost goes to 4 dollars. I have to put some arbitrary number in order to get close to the average cost I want. It creates a discrepancy in our documents. maybe there’s some calculation setting im not seeing and if there is I’d very much like to see it.