They won't be worth a heck of a lot unless you can get them out in working order--maybe they go outside in pieces (it isn't in working order when it is taken apart...) and magically are reassembled when they get back to your shop....?

The heavy semiconductors in them have some copper heat blocks that bolt to the heat sinks. They're a pain to smash open and scrape clean but it can be done. You might be able to sell the parts of the VFD's on eBay as repair parts. This works the best if they are brand name, popular drives like Allen-Bradley.

Hope this helps,
Jon.