Here's a formula to select only transactions from the Prior month only. It is generic and not particular to any data file (apart from the name of the date field):

( 12* Year(TODAY()) + Month(TODAY()) ) -
( 12* Year({Entry Date}) + Month ({Entry Date}) ) = 1

This takes the year and month of today, compares it to the year and month of the transaction date, and only selects those that differ by one month. This is necessary for the filter to work in January, when the prior month is in a different year.