cheapest way to find negative keywords when you only get 20 minutes a week

Cost descending is the right instinct, it's just the wrong second column. Sort by cost but filter conversions to zero first, otherwise your top 50 is full of terms that are expensive because they work. Two more filters worth the thirty seconds: 1) Terms with enough clicks to mean something and no conversions. One click is noise, ten is a signal. 2) Anything containing free, cheap, jobs, salary, diy, or a competitor's name. That list catches more waste than any sorting ever will, and you build it once. Twenty minutes a week is plenty if you spend it on the right rows.