Robert I was wondering if you worked out how high the price of oil has to
Robert be in order for an SPS project to be profitable.

That's the energy component. You also have to know the price of land.

Solar cells on the ground in Arizona might generate about 300 KW per acre.
Call it 6 hours a day (it's more, but the amount decreases off noon), sell
it at $.04/kWH, for $26k a year. A quick google found land in Arizona
("Just 1 hour from Phoenix!") for $2000/acre, and I would guess it can be
had for less, especially in Mexico. If you can pay off the land with a
month's operation, I think the dirt is a good cheap orbit for an SPS until
launch costs drop to $1/lb and you can build a reliable power satellite
in 1 ounce per square foot.

I really wouldn't even worry about putting the land to other uses. Even
the absolute amount of land available does not appear to be a large
problem, as 150 square miles would generate enough power to run California,
and 800 would take care of the difference between day and night operation
across the entire U.S., eliminating those pesky "peaking" issues.

Then, of course, you have to compare transporting energy via thousands of
miles of wire, which is a well-understood problem (buying rights-of-way),
versus transporting that energy tens of thousands of miles via microwaves,
which have seen less deployment due to much larger losses, complexity, and
issues with things getting into the beam (which I think are somewhat
analagous to right-of-way issues).

So never mind space, building and installing cheap solar cells is *the*
problem. Happily, we're only an order of magnitude away (for cells I can
buy today from realgoods.com), and probably a fair bit closer than that
for large commercial systems. Manufacturing problems are much easier
problems than negotiating with despotic thugs and their terrified citizens
in, you know, the mideast.