Starting up a Web company is never easy, but at least it’s not as expensive as it used to be. Instead of buying and maintaining an IT infrastructure, as they had to do in the dotcom boom, startups now turn to cloud server services like Amazon’s. Instead of costly proprietary software, OpenOffice and Google offer cheaper (or free) options. Instead of paying office rent, employees can work from home. And the viral power of social media can bring new customers with little marketing. Open-source projects and the durability of Moore’s Law promise to lower costs even further.

But if it’s cheaper than ever to fund a startup’s growth, why are some Web companies receiving hundreds of millions of dollars in financing? And why are valuations rising quarter after quarter, to the point where some venture capitalists are complaining that certain startups have simply gotten too expensive to invest in? How is it that Web companies are becoming both cheaper and more expensive? Are VCs valuing companies on fundamentals, or following the market’s momentum?

Such questions might seem academic, except that the gap between startup costs and valuations keeps widening. The last six months alone have seen a surprising number of nine-digit venture rounds. In July, Airbnb, a home-sharing startup that had 130 employees, raised $112 million in a round that valued the company at $1.3 billion. A week later, Twitter, which had 600 employees, raised $800 million (half going to cash out early investors), valuing it at $8.4 billion. In October, online-storage company Dropbox, another small company of 70 employees, said it raised $250 million in a round valuing the company at $4 billion. And just last month, group-buying company LivingSocial closed a $176 million round, vowing to raise an even larger amount in the coming months.

There are two key reasons for such outsize venture investments – one strategic and one emotional. The strategic is that startups that have built a loyal customer base and strong word of mouth often solicit big investments to scale up in a nascent or highly competitive market. So, for example, Airbnb is building on its early success to expand internationally and bring in more users. And LivingSocial is looking for a bigger share of a group-buying market that once belonged to Groupon.

“There’s not a lot of value in second place,” Ryan Moore, a partner at Atlas Venture in Cambridge, Massachusetts, told Reuters. “If you have an interesting model, you spend aggressively and build aggressively to win in your category. There are a lot of situations out there where people are betting big.” In accepting a large investment round, a small startup may be banking on ambitious growth, or even preparing against the risk that the capital markets may slow down.