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Larry Ellison isn't the only one with multi-million-dollar real estate and a pleasure-dome on the sea. The billionaires of the tech industry have a proud tradition of ringing up ridiculous bills.

$97 million: Bill Gates spent seven years building his estate. The Gates Mansion has a reception hall, theater, boathouse, and underground garage. The world's richest man also bought a $14.4 million dollar buffer of land around the mansion. And, we assume, to keep Steve Ballmer far from his children.

$38 million: Every now and then, one investment has to tank. It's weird enough that John Doerr thought his Dean Kamen's Segway would make billions; what's mind-blowing is that he thought it might be "bigger than the Internet." It's unwise to make business decisions when still woozy from a Segway ride.

$25 million: Paul Allen funded SpaceShipOne, the private spacecraft that won the $10 million X-Prize. Great investment return, right? But SpaceShipOne is officially the coolest billionaire toy to date. Microsoft's co-founder also throws money at the SETI Institute (surely an extravagant use of his millions; if Vulcans haven't already landed in the Valley, what else could get them to come?)

$21 million: Jeff Bezos is building a rocket-ship launching station in hopes of sending passengers into orbit and eventually building permanent space colonies. The Amazon founder's space base should be finished any month now.

Have any favorites of your own?

Amazon founder's space venture takes shape [MSNBC]
SpaceShipOne [Scaled Composites]
Paul Allen [SETI Institute]
Tour of Bill Gates's estate [US News]
An Inventor Unveils His Mysterious Personal Transportation Device [NYT]