I'll be in Japan for a couple of months, and I figured it might be a good idea to have a phone. I don't have a SIM-card compatible phone, so it looks liek I'll have to get soemthing when I arrive in Tokyo. Would it be worthwhile to just get a pre-paid phone? How much do those usually go for?

Thanks in advance

I think a prepaid phone is your best option, but the calls are expensive. The phone is about 7000 en and the cards are 3000 en. The phone comes with a card that has 3500 en on it - so the phone actually only costs 2500 en ( around $22 US) When you get low on minutes you go the konbini and buy a new card from a machine (they have instructions in English) for 3000 en.

But be warned, everything is in some strange language and even the simplest tasks are a chore.

I use a prepaid when I'm in Japan. Since I'm not resident, I had to get a Japanese friend to buy it for me - my passport wasn't sufficient. I don't know if I just got a difficult store clerk or if that is still the.
Calls are expensive but you can make 3000 yen last if you beg people to call you back.
One slight inconvenience - if you lose your prepaid handset, you lose your number too mind you which means you have to phone/email with new details to everyone you conceivably gave your number out to.

There are several companies that rent phones at Narita Airport. I used to keep a phone in my office for staff coming in from overseas but found it more convenient to have them pick up a rental and drop off when they leave through Narita.
Check the Narita Airport website as they have some info on companies who rent phones.

Renting phones can be expensive though, considering the options I've seen.

The best solution if you'll be here for a few months is probably a pre-paid. But as has already been mentioned, foreigners will need an alien registration card to obtain one. If you're staying on a tourist visa this will mean you will have to get a friend to get one for you. A friend of mine when he first came here on a tourist visa convinced the j-phone girl to put the phone under her name. He might just have been lucky though.