Pi zero W- £10
Sd card - £10 (32GB)
interface- £14 for two buses (if your sensors are all in totally different directions or you want more than 20 of them, upgrade to a pi3 for £32 for and a 6 bus interface for £33)
Temp sensors- about £2 each for waterproof ones (dependant a bit on lead length, but nothing huge)
Cable- 20m is £12 for outdoor UV protected cable, £5 for a load of rj45 plugs
Connectors- £3 each
Humidity sensors- £30 each
Light sensors- I've never found a 1wire light sensor I'm afraid, so you can have one on the pi itself for £3, but not lots scattered around.
Cameras- you can only have one camera per pi, if you wanted 3 I would recommend a pi zero and camera for each camera setup (which adds to your power load a bit)- then you're looking at £10 for each pi, and £25 per camera.
Total (assuming 20 temp sensors, 6 humidity)
sensor setup: ~£360
These are all wireless-enabled, but for ease of setup you might want a seperate router. Any old ADSL router will do- we get a new one every year when we switch ISP- so I'd have thought you could get one for free.
You would want to throw in £20 for shipping everything as well perhaps