Toyota Repair: 96 Avalon starter issues, initial outlay, starter problems

Question
96 Avalon 140,000 miles sporadic (twice every three weeks), transient non-start. Once the engine died while pulling into parking spot, and once continued to run when keys removed.  Drove to dealer and after $540 drove home with new IAC valve put in.  14 days later one nonstart episode, 6 weeks later completely dead.  Dealer sent mechanic to house and I was told need a new starter.  Question: How much more do you think is reasonable to spend given the initial outlay.  What do I do if after replacing starter they tell me it is now something else?  I struggle with the thought of having paid $540 and still have issues.  Should dealer have any responsibility?  If I was told this repair was going to be $750 initially  and it was driving like it was I probably would have elected to drive away and get a second opinion.

Thanks

Answer
Starter problems are not uncommon on these models so I will agree with the dealer that the starter needs to be replaced,unless already done they should also check the battery and the alternator before replacing the starter. The IAC valve they replaced does cause engine stalling but has nothing to do with the engine cranking initially over, it is responsible for keeping the engine idling at the correct rpm.