^ Ooh, lucky you!! I can only imagine what I'd do with $400 to spend at MAC *daydreams... drooling*
I'm not sure if MAC has any stand-alone stores (I know they definitely don't where I am anyway, I usually order them online or wait for the very rare occasions I get down to Brisbane and go to Myer and DJ's), but the cosmetic girls in department stores are usually old hands at helping you out if you're a bit confused or want to try some new things but aren't sure how to go about it. They can sometimes go to extremes a little though (a friend of mine got a Napoleon makeover one day while we were having 'recreational time' during a school trip and walked out into Queen St mall with bright purple and green eyeshadow and spiky false lashes - she looked awesome, for a night out clubbing, but a bit out of place for 2pm on a weekday!) so if you do go for a makeover, explain to the assistant that you want a look that will fit with daytime, and suggestions for night-suitable looks, iykwim. They are very helpful when it comes to matching shades with your skin, hair and eye colours though so definitely ask advice if you don't feel too confident

As far as prices go, I find that they are generally the same throughout department stores etc, but can vary by a couple of dollars online vs dept stores etc - eg, you can pay $32 for a shadow from the MAC website, but if you buy at Myer you might pay $34 (or you might pay $30), kind of thing... they don't vary by much so it's not that big a deal, I don't think it matters
HTH Have fun splurging!!