I pay 3,500 yen / month for "unlimited" service from a Wimax-only (no 3G) router for a 24 month contract. My actual download speed tends to be around 25Mbps.
A bundled smart-phone and 4G service is typically in the 6,000 / month range (24 months).
Whether Wimax or LTE, people are mostly paying about $40/month for service - after accounting for device instalments. That service would also include 3G, which has 100% coverage, whereas 4G is not yet ubiquitous across all Japan. There is precious little incentive not to take the 4G on a new phone contract so most people take 4G now. Of course this fact helps promote smart-phone sales.