Most places charge a �1 a day less for each extra cat i.e. one cat �9, two cats �16, three cats �21. Some do a deal on multiple occupation. However you are paying for peace of mind, so don't just go for the cheapest option. If your cats are not used to dogs, then choose one that does not have boarding kennels, as the noise of the dogs barking could upset them. Do they all get on alright? If they are confined in a small space they may be upset or even fight if one of them cannot get away from the others.
Some catteries are authorised by the Feline Advisory Burea (usually a bit dearer but their standards are very strict to be one of them). Remember you are paying for safety and security, good standards of health and hygiene and the food they are used to - don't think it is just a good way of making money, as cats only cost pennies to feed. They have to cover the cost of council tax, mortgage, insurance, telephone, water rates, staff, buildings, bedding, etc etc. It is not a licence to print money that some people think.
The alternative is to get a 'pet sitter' either to live in or to visit daily to feed etc. Depends on whether your cats are indoor ones though, as they may get upset with a stranger, and try to get out, or as happened recently to someone on here the cat escaped and was missing for several days while the owners were on holiday. If you rely on a neighbour or relative, will they be as reliable as you would want? If they are professional, do they have insurance, are they police checked, or is it someone who has set up for themselves and therefore have unknown experience? Would they recognise if the animal needed to be taken to the vet?
IMHO a good quality cattery would give you the peace of mind so you can enjoy your holiday.