Euro.
€1 = aprox. 0,72£
€1 = aprox. 1'08US$
See current exchange rates.
At the majority of the sites you can pay with a credit card, although if it's not a Mastercard there are many hotels that charge an additional fee. Whatever the case, it's convenient to carry €20 on your person for small purchases, snacks, transportation, etc
Flight: €50-120 (£45-105)
Transport within: €14 (£12) per day per person
Food: €40 (£35) per day per person
Activities: €14 (£12) per day per person
Lodging: €80-140 (£70-120) per night per double room.
In total, you could leave at the end of the weekend for around €175-200 (£150-175) per person, in addition to the cost of the flight.
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