If you plan to visit Japan, you should get ready for its complex Internet situation. There aren't as many free Wi-Fi as you expect. Yeah sure, Starbucks and other coffee and fast food franchises offer free Wi-Fi. But, sometimes they ask you to register your email without actually giving you access to confirm it (they must be thinking only about the people already living there). Lots of services in Japan in general lack the understanding of the challenges tourists from other countries face. So, If you just rely on 'free' Wi-Fis, the chances are you will be disconnected from all the information you'd need for your travel. As a Japanese citizen knowing both ins and outs of the country, I'd say the best way to navigate is to rent a portable Wi-Fi hotspot (which in Japan is called 'Wi-Fi Router'). I've also checked out prepaid SIM available at Narita Airport. But, the information isn't conclusive. Either they offer much smaller data package, or they don't tell you the price on their websites. So, I decided to omit them.
DISCLAIMER: I read and calculated all the deals carefully, but I do not guarantee the chart gives you the exact amount you'll be charged. Please confirm with the vendor before you make your purchase. I assumed the prices on their web sites are before sales tax (unless specified). So, I've added 8% tax to them and reflected on the chart. Insurance is not included. If they charge a courier fee to make it available for a pick-up at the airport, I included it. And, many companies offer extra battery, but it's not included below. Those companies that have branches at the airport, but lack any English contact or website are omitted.
Good luck and have a great time in Japan!