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Keeping kosher, perhaps history's first trendy diet, has an even stricter counterpart during the Jewish holiday of Passover—dietary rules established in order to commemorate and remember what it was like to have to eat on the run when the bread didn't have time to rise.

Despite the simplicity and restrictiveness of the diet, it can actually be pretty pricey to follow it. As Benyamin Cohen reported for Slate a few years ago, an approved can of tuna can cost almost $20 and proper shmurah matzoh—which, unlike your standard grocery store fare, is handmade—can go for as much as $30 per pound.

But why?

In 1991, kosher food giant Manischewitz pleaded no-contest to charges of price-fixing in the matzoh market. But anti-competitive behavior aside, the main driver of high prices is compliance with religious rules and regulations. Whether you're making sure the bread doesn't rise or manufacturing kosher-for-Passover ketchup, doing it right means having a rabbi present to make sure that every step of the process is done in accordance with long-established rules. This "guarding" means that, unlike standard kosher food, the passover stuff needs constant supervision. And, as Cohen puts it: "Someone's got to pay for the rabbi's time." That someone is the customer.

Even centuries-old rules are changing, however—and in the Jewish customer's favor. This year the "kitniyot" ban on rice and legumes during Passover, which has lasted around 800 years, was lifted by Judaism’s Conservative movement, partly to make things easier for vegetarians, vegans, and people with gluten issues. This is also likely to make the holiday less expensive.