Read this on sniper rifles:
http://www.airsoftcanada.com/showthread.php?t=158804
In airsoft; sniping is not sniping like sniping...
Bottom level, reliable gun is a G&G series, they are $200 - $300 for a low level one. It may need some tuning to be a little more effective, so expect a few upgrades, starting with the Hop-up unit. Guns need batteries, batteries need chargers, don't buy cheap or it'll just be a waste of money. Guns also need magazines and magazines need BBs. You'll also need proper ballistic or paintball rated eyewear and depending of your local fields, they may require full seal. I'd also recommend comfortable boots with at least a 6" ankle support. To get into airsoft, bottom level, you'll likely need about $500 after tax, shipping, or whatever may come up. That doesn't include camo, headwear, a mask, gear, a secondary, hydration, etc.