Even with a fresh $3.2 billion in its funding chest, can the "new" Clearwire LLC (Nasdaq: CLWR) really afford to finance its ambitious mobile WiMax plans in the U.S.?

Analysts estimate it could take up to $12 billion, including backhaul costs, to fully deploy mobile WiMax in 50 U.S. markets. Clearwire says it wants to focus on the top 100 markets, using the so-called 4G mobile WiMax technology, filling coverage gaps with 3G. On paper, this would provide wireless broadband coverage for between 120 million and 140 million people and more than 200 million people beyond 2010.

Google (Nasdaq: GOOG), Intel Corp. (Nasdaq: INTC), and a posse of cable companies are pumping in the $3.2 billion, and Clearwire CEO Ben Wolff has said that the company could look for another $2 billion to $2.3 billion sometime in 2010. (See Clearwire: We'll Kick LTE's Butt.)

That adds up to $5.5 billion, roughly half the $11 billion to $12 billion that Technology Business Research Inc. (TBR) analyst Ken Hyers estimates it would cost Clearwire to deploy a mobile WiMax network with supporting backhaul in 50 markets in the U.S.

Hyers bases his estimate on the roughly $1.5 billion per quarter that AT&T spent on deploying its upgraded 3G network. He believes it is "reasonable" to expect Clearwire's build-out costs to reach $1.5 billion to $1.8 billion per quarter for at least 18 months to reach 50 markets.

"That's a realistic number," agrees Phil Solis, principal analyst of mobile broadband at ABI Research .

Sprint Nextel Corp. (NYSE: S) has three markets in a soft-launch phase -- Chicago and Baltimore-Washington -- and expects them to go commercial late in the third quarter or early in the fourth. Clearwire has mostly talked up its WiMax deployment in Portland, Ore., which should go live in the second half of this year. (See Clearwire: In Depth With Barry West.)

Sprint has already been tripped up by the need for faster backhaul for WiMax. The operator delayed its initial Xohm launch in April as it continues to build microwave links for the network.

Neither Clearwire nor Sprint returned calls to comment on the potential costs of WiMax deployments.

Sprint originally said it expected to spend $5 billion deploying WiMax through 2010, with $2 billion earmarked for 2008. The original Clearwire partnership was supposed to shave some of that cost. (See Sprint Goes WiMax.)

Clearwire was spending big even when deploying its fixed wireless "pre-WiMax" network in the U.S. over the last couple of years. In May 2007 documents, for instance, Clearwire revealed capex spending of $74.4 million during the first quarter of 2007. (See Clearwire's Cash Craving and Clearwire: The Big Spend.)

Clearwire's shares dropped 9 percent to $14.76 today after yesterday's ramp-up as Citigroup downgraded the stock to a Sell, citing network costs, coverage, and competition as issues for the venture.

— Dan Jones, Site Editor, Unstrung

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