I just bought a 540 w/ premium and sport without nav and with automatic w/ 55k for $26,000. I would think the nav would add a little more to the price maybe up to $29k to $30K. It could be more if you are buying a certified BMW. I bought mine for my mid-life crisis, 40 years old (or at least that's what I told my wife). I tried 528i and 530i but, once I drove the 540i I couldn't go back. I'll tell you though, in my area there are not that many 540i's and was lucky enough to jump quickly because several other guys were also tring to purchase the same car.
What color in and out are you looking for.