The exact amount is up to you but we have found that somewhere between €120-150 (perhaps with the addition of £10-20 for the outward/return journeys) is more than enough.
Students only need to take enough for lunch (about €10-15 a day), perhaps a couple of hot chocolates if the weather is really cold (€4-5 Euros each), a few treats whilst out and about and some souvenirs/presents.
There will be only be a limited amount of time for shopping and there really isn't a great variety of stores in resort anyway - the Spar is by far the most popular shop! In all honestly students who have taken large amounts of money in the past have struggled to find sensible things to spend it on - smeone bought a drone on the ferry out in 2024!