Water Valve Replacement 2000 LS

We have just been advised that this item needs replacement and the part is around $300 + and the labor about $400 +. This goes between the engine and radiator and supposedly is very difficult to do because of accessability and many hoses involved.

This is what they found the cause to be for our lack of consistent heat in the passenger compartment.

I'd like to know if this sounds right to anyone in the know.

Thanks

Rick