The expansions must be bought in order. Skipping over an expansion will give an error message instructing you to buy previous expansions.
As of July 7, 2011 the expansion times have been permanently reduced. At the same time the prices for instant building have been increased.
With TZ Version 1.7.2 new expansions available. These expansions can be build with the following devices only: iPhone 4, 4s, iPad 1+2, iPod 4th gen. TinyCo told that 09.12.11 at Facebook with the update for version 1.7.2.
These are not the final prices. The prices will change in the next weeks again. As prices change almost daily at the time, we will make an update later. Thank you for your understanding