As it's a secret mission so generally these kinds of information they can't share to public so there is no such a fix amount for this. The factory depends on the work like what equipments they are using.

It has the construction design, launching vehicle, the fuel, the training for the astronauts, the infrastructure and the mission. So mixing all these what cost they make is on them.

Taking another space agency like NASA they spend 10 of billions of dollars.

So there is no such information for the estimate cost of Gaganyaan..