The party said the cost had soared by 23% due to sterling's decreased value since the EU referendum, making holidays abroad now unaffordable for some families.
Calculated using initial figures provided by Asda Money, the Lib Dems said a 10 day trip to Europe for a family of four would now cost £1,609, up from £1,310.
This is because the euro was typically worth 70p before the June 23 vote, but is now trading at around 86p.
Lib Dem leader Tim Farron said: "For some families, this will make a holiday abroad unaffordable this year. This problem is not helped by the failure of Theresa May to say whether she wants Britain to remain in the single market or not.
"I call on her to make clear that she thinks Britain should remain part of the world's most lucrative market and not tie British industry up in endless red tape when it tries to export.
"Such an announcement will reassure markets.
"It will also help people who have worked hard all year enjoy a well-earned week or two in the sun."
During the referendum Remain campaigners warned that the cost of a typical holiday could increase by £230.
Mr Farron added: "Brexit campaigners dismissed the figure as scaremongering but it actually proved to be an underestimate."