Yeah my first service at 800ks was about the $350 mark, I think your dealer is having a lend of you to charge you $500, I took mine to the dealer at that tiime to also have some warranty matters attended to.
You are under no compulsion to use a Triumph dealer, you are able to take it to any competant mechanic hopefully some one you have had good dealings with in the past with. The only time it needs to go to a Triumph dealer is to have warranty issues fixed.
Peter