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 Starbucks Corp. is facing increasing pressure from consumers to switch to organic milk — but be prepared to open your wallet if that actually happens, experts say.

On Sunday, eight national groups — including Green America’s GMO Inside campaign and the Organic Consumers Association — will spearhead a social media day to pressure the nation’s largest coffee chain to switch to organic milk; the participants will post content on Facebook, Twitter and Instagram and use the hashtag #OrganicMilkNext.

Already, more than 150,000 consumers have signed a petition demanding that Starbucks (SBUX) switch to organic milk, according to the groups’ press release.

For its part, Starbucks has already stopped using milk from dairy cows that were given growth hormones, which Green America President Alisa Gravitz called “the right move.” And the coffee chain already sells some organic foods and beverages in their stores.

But critics want more — in this case, Starbucks to use all organic milk in all beverages all the time. And while there are reasons that many consumers might like this idea — some studies indicate that organic milk might be, in some areas, nutritionally superior and have less of an environmental impact than non-organic milk — they’d better be prepared to pay a premium.

“This would have a huge impact on beverage prices at Starbucks,” says Phil Lempert, author and founder of SuperMarketGuru.com — who thinks that it might mean you pay 50 cents or more extra for some beverages.

A Starbucks spokesperson acknowledged in an email that “organic diary is an important issue for some of our customers,” and the company is always looking at its sourcing options. The representative pointed out that organic soy milk is available in its stores around the world.

Why will this hit consumers’ wallets so heavily? Because organic milk costs a lot more than non-organic milk — and for a company like Starbucks that uses so much milk (check out how much milk goes into some of its popular beverages), a move towards organic milk could impact their bottom line, he says.

IBISWorld analyst Antal Neville says that wholesale prices for organic milk are sometimes 50% or more above those for non-organic milk, thanks in part to the fact that organic dairy farmers have to follow certain, stricter criteria for their cows and may need more land per cow than non-organic dairy farmers.

Of course, Starbucks doesn’t have to pass along the added cost of going organic to consumers. Like any other restaurant or food-service company, Starbucks needs to decide how much cost they can pass through without alienating their customers. But past behavior with regards to commodity prices indicates that Starbucks has been willing to let retail prices rise with costs.