Try a quote from a different repairer, if you like the guitar it’s still going to be cheaper than buying a new one, if you’ve not worked on guitars before you’ll probably wreck it.
This kind of job is always more complicated than you might think. 12 strings are notorious for this because of the stress of the string pull. A local luthier, if you can find one, will probably be a better bet than a music shop repairer and most of them are happy to help.