If Liz needs only $4,000 to buy a new $30,000 car, that means she has $30,000-$4,000=$<<30000-4000=26000>>26,000 from the proceeds of selling her old car
If she sold her car at 80% of what she originally paid for and sold it for $26,000 then she originally paid $26,000/80% = $32,500 for her old car
If she paid $32,500 for her old car and the new one is $30,000 then, the new one is $32,500-$30,000 = $<<32500-30000=2500>>2,500 cheaper
#### 2500