Thanks Yihao69, SESTAR, aurigakb, clacko and Buster. I've checked going from SIN via other cities like CAN and from there to PEK. The least expensive is Singapore Tiger Airways but it only goes to CAN and I have to get another flight from CAN to PEK (connection time is a problem, too short or too long).

So I've decided to use NW miles to go "free" (SIN-NRT-PEK). There's 5 hrs layover at NRT but I don't mind since I could use the club lounge. Also I was able to get confirmed business class on the return (not available on the out bound). I was very surprised that the business class is only 30K miles. I'll keep checking to see if a business class seat will become available for the outbound.

Thanks again for the help.