Most people will tell you that your tank will cost you $40-50 per gallon to setup. I'd say that's a good figure to plan on. Also, don't forget to figure in your extra utilities, testing supplies, feed, additives, and such.
Again many people will tell you to get the biggest you can fit in your home within your means. Do it. I got a 58 gallon, and now wish I would have just shelled out the extra $200 for a 90 or 110.
The cost per gallon of your tank and cabinetry is slightly inversly proportional with respect to its size. Your equipment costs are pretty linear per gallon. Stocking your tank, on the other hand, increases proportionally per gallon. So, not only is a larger tank less tempermental, you get more than its share of things to look at compared to a smaller tank.