Is it dangerous to dock a dogs tail after 15 months old?

I do not know costs of that procedure but what I can tell you is it is not dangerous. Seeing how it is hurting her, getting it done as soon as possible will be best. With her being over a year old already it will be a lot more painful and take longer to heal than if it had been done as a puppy. Talk to your vet and tell them the situation and if they offer those services.