How much does it cost to repair PowerBook rubber base in Singapore

We are not Apple, only end users like you, so cannot quote prices reliably. However, all PowerBooks are considered obsolete by Apple meaning they no longer provide service for them in Apple facilities. You will need to find a relable independent service firm. You will get the best recommendations for local service by working through a Singapore area Mac Users Group (MUG). THis page will help you find one:

http://appleusergroupresources.com/find-a-group/

I entered "Singapore" as the country and found two groups within the territory. Either should be able to recommend a good service shop for products Apple thinks are obsolete.