Analysis of cabin bag prices charged by three budget airlines found their lowest advertised rates were not available in the vast majority of cases.
UK consumer group Which? said the carriers "deserve to be called out" over the issue.
It checked the cost of travelling with a bag designed to fit in an overhead locker on a total of nearly 1,500 EasyJet, Ryanair and Wizz Air flights.
These were spread across eight popular routes for each airline, and included peak and off-peak dates.
EasyJet's website states it charges "from £5.99" for a cabin bag, but Which? said it found no price that low among the 520 flights it checked.
The cheapest price found was £23.49, and the average was £30.
Which? said it handed its findings to regulator the Advertising Standards Authority.
Analysis of Ryanair found its lowest advertised cabin bag rate of £12 was only available for two out of 634 flights.
Wizz Air's lowest stated cabin bag price at the time of the research was €15 (£13.11), but that cost was only found twice across 338 flights.
The average fees found for Ryanair and Wizz Air were £20.50 and £28.93 respectively.
Which? said travelling with a bag that can fit in the overhead lockers is "likely to be considered an essential by many passengers".
It claimed airlines' "failure to provide transparent fees" means consumers "don't know what they are paying" until they have gone through multiple stages of the booking process.
Rory Boland, editor of magazine Which? Travel, said: "Our research shows that the tens of millions of passengers who need to take a cabin bag will pay much more than the cheapest price advertised".
"Rather than a few pounds, prices for bags can often be more than the flight itself. The tactics used by these airlines deserve to be called out. That's why we have shared our findings with the regulator," he said.
A spokesperson for EasyJet said its bag options and pricing are "transparent and well understood by our customers".
It added that they allow passengers to "pay for only what they want".
Ryanair said in a statement that the number of flights analysed was "too small to be representative or accurate", adding that its cabin bag prices are "transparent and optional".
In August, Ryanair said it planned to raise bonuses for staff who spot passengers attempting to take oversized bags on flights, from €1.50 to €2.50 per bag.
At the time, the airline's chief executive Michael O'Leary said he made "absolutely no apology" for catching people "scamming the system".
A Wizz Air spokesperson said its prices "vary by route, season and demand", and insisted it is "fully compliant with consumer protection laws".
