say, you have purchase $100/month.
Month 1: $100 @ $10/unit = 10 units
Month 2: $100 @ $5/unit = 20 units
Month 3: $100 @ $1/unit = 100 units
Month 4: $100 @ $1/unit = 100 units
Month 5: $100 @ $2/unit = 50 units
Month 6: $100 @ $2/unit = 50 units
Month 7: sell everything @$2/unit?
Total invested: $600
Total number of units: 330
Price/unit dropped from $10/unit to $2/unit, dropped 80%
If sell everything @$2/unit, you will get 330x2 = 660.
Total invested: $600.
Sell total: $660
Earn 10% when price dropped 80%.
This is the power of cost averaging in mutual fund.