15-20 to drop. It won't wipe out alot of golaiths anyway, depends on if they are in line formation, non-stop moving, and the fact that the only thing they'll stop for is to attack or wait for orders.
Lockdown costs 100 energy to lock out ONE unit, and what would happen if there were 60 golaiths massing on you and only one ghost to lockdown.
Spider mines like i said will bring a golaith to 4/125 HP