in the current market, some observations:
1) sentosa condos, we have seen 2 units which were sold around 2700psf and sold for 1400psf, almost 50% down. feels like this area has been de-rated out of investors' radar. so let's not discuss this further, unless there are any bro/sis who are keen to buy there.
2) the peaked at 3600psf ardmore park as catsick had pointed out, 1 unit has transacted at 26xxpsf. 1000psf off is between 25 to 30% off. however, we need to consider that some units have better views (for now). so for this highest luxury end (but old), in the current market, good buys are at around 25% off peak.
3) the widely discussed grange infinite. ARA fund bought at 2600psf and sold at 2100psf. 20% discount.
4) as we move even further down in both psf and quantum, we have seen that developers have been able to move stock at 10 to 15% discounts. similarly so for resales. e.g. tribeca (there is some discussion in that thread), a 13% discount off the peak for a similar unit. So i'd say, for units around 2000psf, good buys mean buying at 15% off the peak for a similar sized but superior unit.