Considering that he traded his car in, resulting in a swap, saying that he overpaid compared to what you were offered is nonsense! Hash - did you have a 00 MX-5 to trade when doing your figures? The salesman probably makes more commission from selling the car at a higher price - so the paperwork will reflect this - even though he may really have been giving you $18k for your car, and $18k sale on the RSC!
Cosmo - sounds like you paid right on the money for an 03 car, and you did even better when you consider you traded your 00 MX-5.
Make sure (if it's available: ie: less than 3 years since the car was first registered) that you take out the extended warranty from your local Renault $tealer. $900 is a small price to pay for piece of mind in my opinion.
Which part of Melbourne will we see you around?