what cost to replace DC in board powerbook G4 15 inch?

Apple no longer services PowerBooks as the newest ones are now 9 years old. You will need to find an independent repair shop familiar with PowerBooks.

Also repair prices vary from location to location and, as we are all end-users like you and not Apple employees, it would be a slim chance that someone in your area had that repair done recently and could tell you what they paid.

If you can find the part, it is possible to do the work yourself with these illustrated instructions:

https://www.ifixit.com/Device/PowerBook_G4_Aluminum_15%22

Note you'll need select from the three instruction sets based on your processor speed.