Cheapest I've seen was a recent promo by Starwood Hotels offering points at $S2.5 per Mile - this was a bonus on their usual price of $S3.5 per Mile.

Buying miles from another member is not permitted in the program and I would avoid it as (1) you don't know who you are buying from and (2) KrisFlyer could remove them completely and you'd be out of pocket of your cash as a result.