My new water heater just cost $1150.00 because we had something called cross contamination? It took the plumber two days to find the problem. Is that normal?

No it is not normal but cross contamination can mean may different things.

Our plumber took two days cutting holes until he found a bad valve which was allowing the cold and hot water to mix. Hence the cross contamination.
Sound correct? Good thing it did fix the problem but did he string us along? Could this be diagnosed easily?

If the valve was burrieed behind the wall and they had to find it I could understand the charges but my question is what was the valve installed for to start with. What was its true purpose (what did it actually do)? If it has been there for years why did it have to be removed now? I would have had many questions for the plumber who did the work. If the system worked with the valve in it for many years why did it have to be removed now.

I have a valve in my system for over 30 years. It is called a water tempering valve and it is for just one area of the home that I just wanted warm water in never hot. I guess you could call it cross contaminated also but I would never let a plumber take it out.