I set up a test sheet for this, with rows from 1 to 30, if your range is larger you'll have to adjust this formula, but this works:

=IF(SUMPRODUCT(--(MONTH($A$1:$A$30)=MONTH(A1)),$B$1:$B$30 )>100,B1*0.45,0)

The final zero in that formula is what is returned if there are 100 or fewer transactions in the month, because I wasn't sure what you wanted to return in that case.