Cost of replacing rear main bearing; new rings on values

I can't tell you how much because I don't have my estimator function up right now but I'll give you an estimator link for you to use. https://repairpal.com/estimator/lexus/rx300 You mention 'new rings on valves,' I want to tell you that the rings are on the pistons and not on the valves, if you're talking about piston rings, you are talking about a major teardown and rebuild so be sure what you're asking for. I recommend finding a Lexus or Toyota specialist. Here's a directory link for you: https://repairpal.com/lexus-repair-in-albuquerque-new-mexico