willio

Well, the cost really depends on what you plan on building the airport with, how you plan on building it, and how large you plan on making it. Here's what I would think:
TERMINAL:
Balsa Wood: personally, I find it hard to work with and almost impossible to use in a detailed airport. (probably about $30 USD for an entire terminal)
Poster Board with Foam Core Base: this is my favorite building method; although it does not make very detailed terminals, it is easy to work with and looks similar to computer generated architectural models (about $20 USD for an entire terminal)
Herpa Terminals:good looking, but pretty standard for airports (about $200 for an entire airport)
Ground Foil/Base
Spray Painted Masonite: my personal favorite as it is good looking, although it takes a while to build (about $30 for a large airport)
Herpa Ground Foils:nice looking, but you can't "personalize" your airport
Print-out Foils: nice looking; if you make your own, it is time consuming, but it is "personalized", if you use one made by someone else (there are a few here on DA.C that you can download) it is as simple as clicking print ($20 if you home print (as in using photo paper in a nomal color printer), $40 if you have it printed at a printing business)
Accessories:
Herpa Jetways: good all around, might need modifications for highly detailed airports (about $2.50 a piece for a single jetway, slightly more for a double jetway)
Herpa Accessories: rediculously expensive, but, at least for me, they seem unavoidable (about $15-$20 USD per set)
Homemade Accesories: I don't really know much about this, but it can be done
Overall, for an airport at the size of DXB or MEL, I would say at least $300 for terminals, accessories, and the foil. It would cost more if you used Herpa terminals. Again, all of the prices are in USD.

willio