Suppose you teach at a university that charges about $981/credit hour. Your typical course is 3 credit hours and your typical enrollment is 45 students. We're looking at $132,435 in tuition coming in per class. Give lecturers $4000 for the class and you've made yourself a tidy sum. Have them teach four of those courses per semester and you'll take in $513,740 while they take in $16,000.
Looking at the numbers this way, it looks like lecturers should be asking for a slightly larger piece of the pie. Just a thought. You'd think with all the egalitarians in higher ed, lecturers would never have the opportunity to ask. In a sense, they don't. The numbers for adjuncts are too depressing to contemplate, so I'll stick with lecturers.