I flew Malaysian air return singapore to Hanoi (via KL) and it was between $400 and $500 Sing. - good flight, no problems.
Loved Hanoi - my favourite city in asia to date. Went up to Sapa in nth-western highlands area (near Chinese border) and I really enjoyed it - beautiful scenery and hill-tribes etc. (though the overnight train isn't very comfortable). Be careful booking cheap tours in Hanoi - heard some horror stories. We went through Handspan (on Lonely Planet's recommendation) and they were good.