One of the drawbacks of leasing is Optus get to decide that. No negotiations.
You can generally only find out when the lease term is up (24 months) however they might tell you what the current amount is. Note they have a set price list for phones, they don't examine your specific phone and decide on the value of it.
It does seem its not unreasonable. Someone just posted their iPhone 8+ was rated $485. They're selling for a little more than that refurbished on eBay. Your phone seems about $500 new on eBay so perhaps around $350 at a guess? You won't find a definitive answer here though.