Costs in Majorca, for example, are almost twice as high as those in Cyprus, which has the lowest charges among 16 resorts surveyed by Post Office Travel Money.
Other popular spots where charges were particularly high include Faro in Portugal, Split in Croatia and Dublin.
The survey took into account not just the basic care hire cost, but also such expense as satnav hire, additional-driver cover and 40 litres of petrol.
On this basis, one-week car hire in Larnaca in Cyprus was just £295, while in the most expensive - the Norwegian capital Oslo - it was as much as £614.
Majorca was the next most pricey - at £514 - while Split was £509, Faro £503 and Dublin £482.
After Larnaca, the best value destination was Luqa in Malta (£317), followed by Marmaris in Turkey (£325), Copenhagen (£335) and Salzburg in Austria (£362).
Post Office Travel Money said its survey of 2,106 UK adults showed that in hiring a car, 56% failed to take out excess waiver insurance that reduces the excess payable for accidents or theft to zero.
Even more (76%) did not take out insurance cover for tyre, undercarriage and window damage.
Andrew Brown, of Post Office Travel Money, said: "These figures make it clear that it is a false economy to cut costs by sticking to the basic cost when booking car rental online. We advise against taking a risk that could cost hundreds of pounds if your hire car suffers damage or is stolen."