If out of warranty and doing the service by myself, would only change spark plugs/tail oil, take thermostar out, inspect, clean, change, whatever needs. If still peeing properly, leave impeller as it is, this item do last much longer than 300 hours if flushing your engine frecuently after use. If under warranty, just take the wallet out along with a gun and ask for a decent discount!!
That huge cost must include definitely a engine scan, as those 2 stroke engines use ECU and lots of sensors to perform as expected.