Gotcha, though if you go back to them with a "It will cost $crazy to store them as is" or "Compressing them will only put off $crazy for $smallamountoftime" then that may change
Anyway, where does this all happen? I assume it's part of the backup process which means you probably want automated, is it Windows or Linux?
Either way you should be able to script something using something like ImageMagick, I believe it supports compression in TIFFs so you should be able to run all the TIFFs through a conversion before they go off to where they're supposed to.