24x18x18 Viv,substrate,water bowl,2 hides,ceramic bulb and holder plus wire and plug,bulb guard to stop any chance of burns,a Pulse thermostat which is the most important piece of equipment.This allows you to set the temp in the Viv and prevents any overheating.any heat source should always be run through a thermostat.
You will not get the heat needed from a heat mat.mats are contact heaters that go outside the Viv so it wont be able to heat the inside up enough for a ball python.
Thermometer to keep a visual check on the temperature.Then you just need any additional decor.My ball pythons love to climb so you could add a thick branch or 2.Also get a lock for the sliding glass doors to stop it escaping.They are heavily bodied snakes which can easily slide open a sliding glass door.You dont need any lighting for royals unless you want to fit a very low wattage bulb (so not to effect the heat gradient too much) for viewing but remember this must also be guarded and only needs to be on for daylight times so the snake has a natural day/night cycle.Then you just need your snake and frozen food.
If you do the job right and use the correct equipment i should think £150-£200 should do it.Just dont skimp on things ,especially the thermostat.You must provide the correct enviroment for your snake and he will be healthy and happy for years to come.