Cost is the most important aspect of any good architecture because it is cross-cutting across the other 4 pillars. If you have enough money, you can build anything, but this is not always a good thing. You can end up not making conscious decisions about where you spend your money and can end up over-engineering a solution to its detriment. Most projects (and hence architectures) will almost certainly be constrained by budgets. However, cost not only acts as a constraint on the architecture, conversely it can also help reinforce any of the other 4 pillars.
The Final Pillar: Designing for Cost
