I've used IACE (www.iace.co.jp) on several occasions - prices are usually excellent AND they allow me to pay from London with my visa card (you need to fax passport info etc to them to do this).

Generally I've used them to buy tickets from NRT to LHR, but on two occasions I've bought flights to the US (LAS & JFK). The JFK flight (November before last) was MUCH cheaper than the price you quoted - but then, it was November. August is usually much trickier.

I'd also suggest you move those comas over:
15,4000 YEN 19,5900YEN

My first thought was that you'd found yourself a hell of a deal!

Then I realised you meant:
154,000 YEN & 195,900YEN. Makes a HUGE difference.