Honestly, I don't know.

I consider myself quite a logical fellow, and now I don't see how spending that amount for an item that holds almost no resale value makes sense.
Alot of times most items you buy altho there 'resale' value...but what are the chances of you really selling it?
I'll say spend a comfortable amount bah (and also consider if you wanna give a 'wow' factor to your SO's friends due to the big diamond.) I believe the average proposal ring size in SG is 0.5-0.7, if you wanna go for an above average yet still not overly priced, go for 0.85-0.89. Or else 1+ if you want a 'wow' factor. haha.
Another good gauge will be around 1.5 months - 2 months of salary for an average or go as high as you feel like if you will love to spend more.