The main three mobile providers (Docomo, Au, and Softbank) are all very similar in terms of plans and monthly fees. There's some discussion (as anywhere) about who has the best coverage in a particular area, but otherwise I think they're almost identical.
There are MVNO services but I have found it hard to get a handle on how to sign up for them. The best summary I know of is at:
http://www.japanmobiletech.com/2014/01/updated-comparison-of-b-mobile-products.htmlThe big three mobile providers will all offer you an unlimited mobile data plus XX minutes of voice for about 7000-8000yen per month. (Say $70-80.) If you don't sign up for a data plan I believe you can get a voice-only plan for 2000-3000yen per month, but then you will be charged per packet for any data you use. Email, texts, and other data services will all lead to packet charges so you would really have to be extremely careful with your data usage to try to keep your monthly cost down. Plus you will probably find as you make friends and acquaintances here that it will be difficult to explain why you don't text or use Line for online chats (or whatever comes along to replace it).
Some of the plans and SIMs covered in the MVNO link above include both voice and a limited amount of data, though usually not at the full LTE network speeds. Again, if you really don't use much data and are careful to keep your usage down, you could probably do well with one of these plans and pay maybe 3000-4000yen per month.