A cheaper solution uses an iPhone and iPod Touch (which looks like the iPhone, but is just an iPod with a big screen and wi-fi capability). BulletFlight software is available at the iPhone software store, and it performs the calculations needed to account for atmospheric conditions (wind, temperature, humidity, altitude and barometric pressure) for long range shooting. The output tells you how many clicks to adjust your scope to make the shot more accurately. Before use, you input basic data like rifle type and bullet weight.
Now a highly experienced sniper can do this in his head. There are similar dedicated devices that cost about $7,000. But BulletFlight is affordable, does the job and appeals to inexperienced snipers, hunters, recreational shooters and those who are just curious. The software costs $12. If you want to actually use it, you need to buy a protective case ($50) for the iPod, and a device ($25) that attaches it to the rail found on most sniper rifles and high end hunting rifles. So for shooting purposes, the entire kit costs you about $90. An iPod Touch (8 gigabyte version) sells for under $200.
There is some concern that the light from the iPod Touch screen might give away the snipers position at night. But you can set the light level on the Touch, so it probably won't be a problem. Besides, you are using BulletFlight (or the competing iSnipe) for long range shots (300 meters or more), so the light from the display won't be a major risk.