the replacement cost for the trim ring is somewhere in the neighborhood of $10-12. I find that what contributes to the chrome peeling is allowing the fuel pump nozzle to lean against it when filling the tank. if you hold the nozzle vertical, and don't put pressure against the trim ring, it lasts much longer. my bike is on it's second replacement for that. the original and the first replacement, I just filled up normally. after the 2nd replacement, I am very careful to hold the nozzle up. I was going to make a small pvc 'guard' that would fit inside the refuel area, but haven't gotten around to that yet.
now if you're referring to the fuel cap itself, that bugger is $94. what is the real killer is the fuel gauge, $270, which doesn't make any sense to me since you can buy the whole kit, fuel gauge and flush mount cap for $210 (or less from a 20% off dealer)