JR Pass or not

I'm not sure if I should get myself a one week JR pass or not when I'm going to Tokyo in October.
I'll be going once to Nagoya (one way), at least once to Chiba and back, probably to Kamakura and back and use the Yamamote Line quite a bit during this week.
If anyone could give me an advice I'd be very grateful (I bet somebody here knows how to look up fares better than I do). Thanks in advance.

A round trip to Nagoya by regular tickets costs about 22,000 yen. A round trip to Chiba costs about 1000 yen. To Kamakura about 2000 yen. Urban JR transportation costs about 200 to 600 yen per day.

Look up the fares here

http://www.hyperdia.com/cgi-english/hyperWeb.cgi

Based on the places you are going, a 7 day JR pass would not be good value.

Tokyo-Nagoya 10,780 Yen, one way.
Tokyo-Chiba (if you mean the city) 620-1,120 Yen, one way
Tokyo, Yamanote Line maybe 600-700 Yen a day. Although you may find using the subway more convenient for some journeys for which the JR Pass would not be valid.
Tokyo-Kamakura 1,780 Yen, round trip or 1,970 Yen using the much more useful Kamakura Enoshima Free Kippu:

http://www.japan-guide.com/e/e2358_004.html

Thanks a lot. That's what I thought. :)