Since it is free to SPAM, all a spammer needs is one sale, and they have made a profit... And perhaps they don't even ship anything, but just accept the money.
I suspect many of the "Get rich on the internet" schemes involve spraying out as many messages as possible. If only one in a million is greedy enough to jump at the offer, it still costs nothing to generate the "sale."
I read the other day in an internal Postal Service publication about how a Postal employee stopped someone from sending off a $4,000 international money order to a spammer... and this old couple had already sent $12,000 in order to pay the taxes on their foreign lottery winnings...
So someone made $12,000 with zero cost... and may get free room and board for a few years too!