Your local field should give you a fill if you ask nicely, they most likely will not charge you or if you are playing, as said above fills are usually included in your green fee. But if you go mid-week for a fill, the going rate is £2-£3
Even with air-safety training, you may not be allowed to fill at a walk-on or regular rental day. I have an air safety pass to self-fill to 3k or 4.5k for example, but it's only good for big games like the Super 6.
Some people go for a fill just before the end of a day's play, so they have air to tinker and plink in the back garden