If the car is a heavily modified performance model, not much.
If the car is a performance model in restored condition, a survivor or near survivor, a missing fender tag could be the difference between a sale and a no sale.
So the loss of value really depends on who the car is being marketed to and the buyer.
Some buyers, simply won't buy a performance model with a key components like a fender tag missing. In a case like this the missing tag could represent a loss of 100% of the cars value.
The next guy that comes along may not care at all.