The mixing valve is about $280 plus labor cost. Mine was around $500 installed. If you can do it yourself money saved, but most likely you'll have to at least partially remove the water heater to get at it.
Some say you can pick one up at Home Depot for less. You just need to assure the fittings are the same.
But ... Be warned ... It will still - always - have a whine most likely. Ours have. This new one does too. If you have no hot water or the flow is minimal it's a sign it's starting to clog. They claim it can be removed and cleaned if you're so inclined.