Actually I was referring to the cost to the user who wishes to protect themself. Signal is designed to be fairly low "cost" i.e. easy to use. If they can find a

Actually I was referring to the cost to the user who wishes to protect themself. Signal is designed to be fairly low "cost" i.e. easy to use. If they can find a way to make it more secure without making it harder to use they should and probably will do it. For most cases, the benefit of privacy probably doesn't warrant going through more trouble than this. For those cases where somebody needs more protection, there's a way to go through a little more trouble to use Signal more diligently (agree with important parties not to change keys for a period of time). To your point though: the cost of executing MITM doesn't just include the equipment, it includes showing ones hand by being discovered.