In second what markystar on that - those prices are way too low !
I didn't check much on the prices when I bought my DoCoMo phone and honestly I didn't really care too much at the time.
I bought an expensive phone ( ~55,000 yen ) and signed up for a plan somewhere around 7,000 - 10,000 yen a month.
Since I lived far out way up in the mountains I signed up for a full phone/msg plan plus internet modem for my laptop ( through the phone ).
After barely using my phone for much more than a few messages and checking my email on my laptop once or thrice I decided I probably should downgrade to save a bit of money though.
However, even after quitting the most expensive features I still ended up paying around 5,000 yen a month.
To be honest I am really surprised at how much the Japanese apparently have to pay for their phone subscriptions.
( For comparison I pay around 500 - 1,000 yen a month for my Danish phone subscription. )
I don't regret signing up for my phone though - a keitai is an absolute necessity in Japan !
I absolutely love my phone, and my mother did sleep better at night knowing she could always get a hold of me even If her son was all the way across the world.
Best of luck !
/Clement
PS: I found DoCoMo's international help line very useful and always helpful. This is absolutely priceless when you live too far from the nearest shop to go there just any time you need something - they even rearranged my subscription plan over the phone !