When you think about buying Japan Rail Pass, you may compare the price of Japan Rail Pass and a single fare. But it is very hard to get an exact fare because the fare rule of Japan Railway is so complicated. So I will give you some examples and consider which way has greater value.
Narita airport to Kyoto for round trip
Total Cost: 31760yen
As long as you travel within 7 days, Ordinary 7 days pass (28300yen) is cheaper than a regular fare.
Narita airport to Sendai for round trip
Base Fare: 13660yen
Surcharge: 3220yen (Narita Airport-Tokyo), 9620yen (Tokyo-Sendai)
Total Cost: 26500yen
Ordinary 7 days pass (28300yen) is more expensive, but I go for the pass. Because the pass can be used during staying in Tokyo and Sendai. And also there are some popular sightseeing spots, such as Matsushima or Yamadera, around Sendai. You can use your pass to travel to these spots.
Narita airport to Hakata (Fukuoka) for round trip
Base Fare: 25320yen
Surcharge: 3220yen (Narita Airport-Tokyo), 16560yen (Tokyo-Hakata)
Total Cost: 45100yen
It is exactly same price as Ordinary 14 days pass. If your trip is in 7 days, of course 7 days pass is a great deal!
Even if you travel more than 8 days, the pass is much easier to handle. And also you may have an extra trip.
Anyway if you go to Kyushu island, the pass recover your cost easily.
Narita airport to Sapporo for round trip
Base Fare: 25880yen
Surcharge: 3220yen (Narita Airport-Tokyo), 12000yen (Tokyo-Hachinohe), 2400yen (Hachinohe-Hakodate), 3030yen (Hakodate-Sapporo)
Total Cost: 46530yen
Ordinary 14 days pass is much better choice obviously. If you have an itinerary that includes Hokkaido island, choose the pass.
Fare table of major sections
I show you some base fares. Please use these fares to compare to the price of Japan Rail Pass. But this is just standard fare. If you have Multi-city travel, the fare may be cheaper. These following fares are based on Shinkansen and daytime limited express trains. (As of April 2012)