I realize there's value added to that it's specifically targeted spamming, but, yes, $1 per message is several orders of magnitude more expensive than normal spamming.
$1 a message is a one followed by the zeros you get from your cat playing with the zero key times more expensive than bulk spam.
Let's use some vary pessimistic estimates.
183 billion spam messages a day * 1% make it to inbox / 10,000 people world wide making spam / 5$ per person per day = 36,600 messages dollar. Which need to be vary garbled to make it though the inbox so they tend to be instantly ignored by most people.
PS: I have seen more detailed estimates that suggest each read spam message costs .1c to 10c depending on how targeted the campaign is.