You might want to look up Sum in Filemaker help.
Sum expects a list of fields in one of three formats:
Sum ( Field1 ; field2 ; field3 ...)
Sum ( RepeatingFIeld ) // this sums all the repetitions of a repeating field
Sum ( RelatedTable::Field ) // this sums Field from all the related records
What you have is the equivalent of Sum ( 5 ) and the sum of 5 is 5.
If you want the sum of all the records in your found set, do it this way:
Define a summaryfield, sTotalSalesPrice as the total of Sales Price.
Define a summaryfield, sTotalPropertyNRA as the total of Poperty NRA
Then define your calculation field as sTotalSalesPrice / sTotalPropertyNRA
I played with it for about an hour and came to your exact conclusion. I was not familiar with Summary Fields and now I've learned.