I'd agree with Rob, you're insuring yourself from loss of income and replacement of the tools that create that income. I'd compare it against newer tools of same horsepower and throughput. Your history of business should also help serve as proof of your throughput. Also any receipts for purchases and maintenance to show the machines are in "like new" condition will help justify any claims you have to replace them with new machines. Just because a machine is old, does not mean that it can't out perform newer machines too, just harder to justify if you don't have some backup documentation.
To a small child, the perfect granddad is unafraid of big dogs and fierce storms but absolutely terrified of the word “boo.” – Robert Brault