That's a crazy price, but main dealer parts are often a joke. Is it just the backbox you need to change? If so, look for a second hand Prodrive backbox, they're slightly louder than standard but very quiet compared to most aftermarket systems.
https://www.ebay.co.uk/itm/SUBARU-WR...AAAOSw4QdabIPV
https://www.ebay.co.uk/itm/subaru-im...gAAOSwxxdZZAfR
