Toyota Repair: leaking valve cover gaskets, toyota avalon xls, timing belt replacement

Question
I have a 6 cylinder 1999 Toyota Avalon XLS with 73000 miles.I just took my car in for inspection and got a $1300 bill for replacing valve cover gaskets ($422). The Toyota dealer also charged $ 868 to replace my timing belt that was soaked by oil along with the water pump ( I know why they did the pump). My real problem is this:just 6000 miles and 12 months ago,the dealer said I should replace my timing belt.That was a $340 charge. Should they have done the valve seals then - could they have seen they were going bad? I think they should give me a credit for the initial timing belt charge. They did not replace the thermostat as noted in your responses.Is it normal for valve seals to go bad in 70000 miles? How do they see this?

Answer
You mentioned valve cover gaskets and then vlave seals, were the valve stem seals replaced? Or only the valve cover gaskets? The timing belt recommendation to replace it is 90,000 miles. In this case there was an oil leak or water pump leak which required timing belt replacement.