What's the current COV market rate for resale 5room? (Built 1994, 126m^2, non-matured estate, near to mrt)?

Different estate have different prices.

So depends on your area.

But those transacted prices if you see are in a similar range forms the bulk of valuation price that will be taken into consideration.

It's not possible to know what's the COV as only buyers will know.

Those information you see in the HDB Website are transacted price and you may not know if consist of COV or without COV.