I am really not sure as to what to value the birds. I know I paid $175 for a trio of Citron d'Uccles last time, but that was over a year ago. I am praying they will be found, can't imagine that there are many SQ Citron d'Uccles around.
I agree that someone you know or who is one of your neighbors might be the culprit. Who knew you were going to be away?
One thing I would like to add to the excellent suggestions you have received is a walk (block by block) in your neighborhood, listening carefully to the sounds you hear.
Your flock may have just been "relocalized" to someone else's backyard, and you may hear your birds calling for help.
Do you have any pictures of your birds? If so i would post an add on craigslist with a few pics of your birds. Let the general public know your birds have been stolen. Maybe even put a add in the local paper. there is still some good people out there and some one knows where your birds are.
If you can offer a decent reward someone who knows something might step forward to squeal........
perhaps a plea to local media since they were a unique group of birds?
I'm always amazed when I read stuff like this. Disgusting chicken thieves. I'm so sorry someone did this to you, and I hope you can find whoever did it.