How is your underlying data set up? 1 row per claim? 1 row per defendant? It sounds like you have 1 row per defendant, if $50,000 value is calculated at a row level--which is where basic filtering takes place.
Can you mock up some dummy data, or share a packaged workbook?
You need to set up a new calculated field that sums the total reserve and then configure it to compute using defendant, for each claim separately.
Create a calculated field with the formula
TOTAL( SUM( [Reserve]) )
and add it to your view. From there, change the 'Compute Using' option (right click on the field's pill) to Defendant. You should now see the total reserve per claim.
Now copy this field to the filters shelf (hold down Ctrl as you move it with the mouse) and select At Least 10000
Now you should just see Claims where defendant's reserves total atleast 10k.
I have attached a sample workbook doing a similar thing for you.
Book1.twbx.zip 1.1 MB