how much should it cost to replace the water pump on my 2005 lexus LS430 ?

Hello, I would not recommend guessing on replacement of a water pump for this concern. I would strongly recommend having a proper diagnostic test performed to see what is happening. Whereas it is possible that a water pump is failing to work, it would be more unusual than other more common causes, like a cooling fan that is not working, or even an air pocket that remains in your cooling system following replacement of the thermostat. Further, it could be a sign of an internal engine coolant leak. Therefore, it's best to check rather than throwing good money after bad.