The financial services company claims around 11.6 million people are planning to go on holiday this year - which could lead to a shortfall of £465 million.
A poll of 2,001 UK adults between 21 and 23 July carried out by Opinium found that around 680 respondents (34%) set aside £78pp for pre-departure tests, yet the average price is £118.
This means tourists could be under-budgeting by around £40pp.
Nearly three quarters of Brits (72%) admit they have not budgeted enough with over a quarter (27%) setting aside less than £50pp.
The average cost of testing that provides results in 12 hours or less stands at £171, compared to £114 for results in 24 hours and £100 for tests in 48 hours.
Two in five (40%) of those planning a holiday abroad have found out how many tests they need to take for their trip and return home, while only a third (33%) have found out what additional documentation they need to travel.
Katy Lomax, chief experience officer at Capital One UK said there is still "a lot to consider" when travelling abroad, "especially around testing".
"There is so much pent-up demand for holidays, and many will be excited to go away for the first time since the pandemic hit," she added. "Our research shows that where you get a test and how far in advance you book it can have a huge impact on the price, so it’s definitely cost effective to be organised."
"When booking a holiday, it’s worth remembering that paying on a credit card could offer more protection than using a debit card or cash, which in these uncertain times, can be very reassuring."
