Filter BEFORE aggregating

[thisisfun]

Please share a link if I’ve missed this somewhere… can’t find the answer.  Seems like a pretty basic need/question.  I have a lot of detail data, and trying to filter BEFORE the aggregation.  For instance…

PK1  Socks   29.00

PK2  Socks   28.00

PK3  Socks   35.00

I am trying to aggregate all sock spend where the individual sock spend was under $30.  Without having to pull in all the details.  Seems like I’m either doing something wrong or perhaps the model I am using is just not set up to allow for this somehow. When I try to use a custom filter or measure, the field I’m trying to use for either isn’t available - ie, not in the list to choose from.  What in the world am I missing?