10k was an figure myself and a couple of mates came up with for the setup of a whole CQB site in a warehouse from nothing; including insurance for the first year, hire guns, masks, overalls, materials for building, BBs, batteries, magazines, cardboard pyro, radios, first aid equipment, etc
That figure DIDN'T cover staff or rent, purely setup costs.
Bear in mind, a halfway decent hire gun is gonna cost ~£100 (that's with the VAT reclaimed), hi-cap magazines are a tenner a pop, batteries are another tenner... so that's £120 per hire gun, 20 hire guns and you're already down £2,400. Halfway decent masks (CQB = fog) are gonna run for another £20-30 each.... you get the idea! Either way, not a cheap venture to start. Setting up a woodland site is significantly quicker and easier, as you don't need to worry about buying materials and building all the cover etc straight away, a couple of weekends with a spade and a saw and you can have some halfway decent stuff going straight off.