custom text in accounting cell

Hi there,
i'm new here and didn't find anything like the problem in search.
I am trying to get "AUD" to appear in a cell that is a =SUM

"AUD "_-$* #,##0.00_-;-$* #,##0.00_-;_-$* "-"??_-;_-@_-

[A1] 500 AUD $500.00
[A2] 1000 AUD $1000.00
[A3] =SUM(A1-B1)
Result of [A3] -$500.00 i would like it to read AUD-$500.00 or -AUD$500.00

Thanks in anticipation.
Neil