Singapore's car resale value is indeed tough to estimate. too many factors to consider. As a general guide, can try this -

1. Current paper value - can get this info rather easily from LTA, https://vrl.lta.gov.sg/lta/vrl/action/pubfunc?ID=EnquireRebateBeforeDeReg
You'll see ur PARF and COE rebate. total will be your paper value.

2. If ur car is less than 5 yrs old Jap car, can add around $5-$8k to your paper value. This will be the estimated amount u get if you scrap your car. So to sell your car, you can maybe add another $2-3k above as your selling price.
If your car is conti, your body value will be higher

An even easier way is to just call and ask any dealer how much they willing to offer for your car. do your research, and try to sell your car at $2-3k above what the dealers offer you