I saw this on a bumper sticker on a car when I went to Starbucks and I thought it was pretty clever, but it may be even better than I thought, if the author had a second meaning in mind as well, so that the bumper sticker has a double meaning.
The first meaning, the intended meaning, is probably this: People complain that education costs are skyrocketing, with private colleges charging now like $50k a year or so, and professional schools even more. I think medical school now costs like $100k a year. Education costs have been going up much faster than inflation. Students have to take on larger and larger loans because more and more parents are unable to pay. So education is very expensive, this is obviously true. But the alternative, which I guess is supposedly "ignorance", is even more expensive in that if you forego formal schooling and lack a degree, you will give up even more in lost income. Because "education" is so important today in terms of lifetime earnings, more so than before, as we can see from the growing income gap between those with lots of education credentials versus those without. So lack of education, or "ignorance", is very costly, or "expensive", in terms of lost income opportunity. I think that must be the intended meaning of this bumper sticker. Or just that stupidity and ignorance can end up costing in mistakes made that could have been avoided with proper knowledge. Something along those lines. And it's pretty clever! Because we don't often think of Ignorance as something expensive to acquire, but rather as FREE! Just do nothing and don't go to school, and you will attain Ignorance. Easy and free. Yet as this bumper sticker points out, it's really not free in terms of the consequences of ignorance.
Yet there may be another meaning to the bumper sticker as well, which is this: Education is indeed expensive, requiring several years of expensive tuition and expenditure of one's youthly time and energy. But at some point, formal schooling does come to an end - 4 years college, 4 years medical school or 3 years law school, or whatever post-graduate education you choose - it involves several years and perhaps hundreds of thousands of dollars. This is expensive. But how about Ignorance? It's even more expensive. Why? Remember that Socrates was the wisest man in all of Athens in his day, during the Golden Age of civilization, the height of high culture and wisdom, Periclean Athens, and therefore the wisest among the wisest of all time. And remember what he said at the end of his life, that after a LIFETIME of nothing but thinking, study and inquiry, a LIFETIME (not just a few years in his youth), the end of wisdom consisted in nothing more than becoming aware of his IGNORANCE. So it took the wisest man of all time the complete devotion of his entire life, which is WAY more expensive than just a few years and a few hundred thousand bucks, to obtain what everybody else has free from birth - Ignorance. The only difference, he said, was that where as others were ignorant but didn't know it, he was aware of his ignorance. Yet nevertheless, he must indeed have been ignorant, otherwise his awareness of his ignorance would have been false (i.e., if he thought he was ignorant but was not). So Socrates did acquire Ignorance, at the cost of his entire life (and it was this "Ignorance", and him repeatedly shoving it in other people's faces his whole life long (i.e., other people's ignorance that they had been unaware of and did not appreciate becoming aware of via Socratic questioning) that ended up costing him his life literally, and being put to death). A few years of school is cheap in comparison.