How much would you estimate the cost of, I guess, adding shirring to the back of a dress like this? The max say 87cm, while mine are more 94. Is this something simple to fix?

Also, how much does adding shirring to an unshirred item usually go for? Assuming I'm not going to the most expensive, worked on the Queen's wedding dress type of tailor and more the cleaners type of altering. And where do they get the right fabric? Or do they just match whatever's closest to the original fabric?

Sorry for all the questions, I don't know a lot about seamstresses.