I've heard of people charging a percentage of value as their cleaning charge, but not adding a percentage to the cleaning charge. Tell me there's more to the story!?
If they are truly adding this charge to their current price, it seems to be an audacious add-on scam. It costs them nothing and adds no value to the order, and no matter what value the customer says, they get to tack on an extra charge. If I'm a customer this would chase me away. If I'm a crafty customer I'd say the rug has only sentimental value and let them try to put an extra charge on that.
If their stated purpose is keeping the customer from inflating the value of the rug, I don't believe that. The value of the rug only comes into play as part of your risk, and only matters if you do some damage. At that point you get an outside appraisal or two to establish value anyway, because what the customer paid or thinks the rug is worth isn't what you pay.
PS: Ed Cross did a great series on legal tangles called "Winning the Slime Wars" which told about how the perception of your company makes the difference between winning and losing. I'd like to see how this policy would play out in court.