Geez man. These things do cost quite a bit, but they don't have to be so expensive unless you have monies to throw around. 😛
I'd say on average a young entrepreneur needs around $1,000-$10,000 for their initial web start up. With that said, many successful projects have started with just around $300-$500. And we should note that many of those that have reached the $10,000 range needed to build physical components (such as an offline gadget) in addition to their website.
Thus, there's no need to spend hundreds of thousands of dollars to start a social network. It'll be interesting to see where you'll get your investors from, since social networking isn't a hot spot for investors. The trend seems to be shifting towards the convergence of personal well-being/heatlh with the web.
In short, there's no need to spend so much money for starting a website. It'll be expensive but not that expensive.