Wow... well, you know.
The costs will be a lot higher to go through all their systems and ensure there isn't any malicious files and malware put on them. It's not uncommon for criminals to give you your network back, with some attached malware/backdoors hidden very well throughout the network. Especially on network devices and DNS, where admins don't typically keep a close eye on.
Hey, but you may have your data back and you managed to encourage and provide motivation for more criminal activities like this. I'm sure those taxpayers are happy with your decision.