You have to be in touch with your CSP Partner / Reseller developer and ask them to code for you.
Basically a function might be created that sum up all UnitPrice value per Item and then it divide the value for the number of entries.
Your developer have to evaluate, then, if these has to be added as a calculated field in the table or calculate the value on the fly (it depends what use you want to do with this field - if this is for this report only then a function to calculate might be the good choice -).
