Start by looking at roughly what it costs to keep your car even if you don't use it. Tax, insurance and servicing need to be paid and if you add in the cost of the occasional maintenance/repair (tyres, battery, exhaust etc) it can soon add up to £1000, depending on the age and size of your car. You can buy a lot of taxi journeys for that. The tricky bit is how you use your car for holidays. If it's just a means of transporting lots of bit to the destination and back again then hiring is OK. If you tour, hiring might work out expensive as some car hire firms have a mileage charge.