Plane spotting is what got me into photography in a more serious manner. It is really what prompted me to buy my first DSLR.
The name of the activity is more or less self-explanitory. However, there is more to it than just spotting aircraft. There is a whole community of spotters around the world. The more serious/traditional spotters, will record the tail numbers of the planes and any other details of that aircraft.
The less traditional spotters, like me, will enjoy taking photos of the aircraft only. In addition to being aircarft fanatics, we are also hobbyist photographers, some even sell their work or get their work published. Personally, I have one of my prints hanging up at the home of an Air Transat pilot.
Where the money factor does come in, is with the lens. Personally, i would not bother with anything under 200mm. Get a little 55-200mm and you will be happy. This will give you a good range. If you have a few hundred dollars more, dont bother with the 200mm, go for a 70-300 or a 55-300. Whatever telephoto you do get, make sure it has the ability to auto-focus on your chosen camera.
Some people get more serious and bring along a ladder to get above the fence line. ( In regards to the ladder, DO NOT put it right up against the fence!! You will have security or even the cops surround you in no time!) People also bring scanners to listen to ATC.
Whatever you do bring along, make sure it will not get the attention of security/police and respect the rules and the surroundings. Don’t be the one that ruins it for everyone else.
Here are a few websites that may interest you if you want to pursue plane spotting: