1 - You should be able to find a spoiler for 2-300 in the aftermarket parts shops. If you want to go for NISMO parts, obviously it will cost a lot more. Allow for 1-200 if you want to paint it. Installation maybe another 100. People here are fighting for sales so you can always ask for free installation when you’re haggling on purchase price.
2 - Depends on the spoiler. Some are stuck on, some are screwed on. Windscreen bonding glue is great for sticking them on, almost as strong as screwing it on, but it will damage your paint if you want to remove it. Bolt on spoilers can also be stuck on this way. Either way, whichever option you choose, removing the spoiler will cause some sort of damage. Personally, I like to screw and glue. Double security.
3 - Yes, your gas mileage will be decreased but the amount will be negligible. You probably won’t notice. A set of roof bars would have a more noticeable impact on mileage. Anyway, you drive a patrol, when did gas mileage become important? 😊