If you have a field in the table and add it as a measure in the summary, then Monarch will just add up all the averages, which, as you spotted, makes very little sense.
I guess your table contains two other fields, Sales and Quantity, from which you have calculated the Average Price on each row of the table?
Then in your summary, if you edit the Measure expression to be Sum(Sales)/Sum(Quantity), you should get more sensible numbers.
Hope this helps,
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