There are two problems I see with a single 8GB module:
1. 8GB modules are more expensive than two 4GB modules, often more expensive than four 4GB modules.
2. A single module would not be run in dual channel and would thus lower performance slightly compared to two or four 4GB modules.
Your best bet would probably be to get two or four 4GB modules for now and wait until 8GB modules become priced reasonably. Considering how that may be a year or a few years you might be ready to get a new computer by the time the price comes down.
Also worth mentioning is that 8GB DDR3 modules may never be cheap and you will have to either deal with the price or wait until DDR4 comes out and makes 8GB modules affordable.