> My past calculations lead me to believe that a 10GW SPS
> would cost $70B; [...] That assumes that everything is
> launched from earth at a reasonable estimated (and
> industry supported) RLV cost of $500/kg,
a) LEO?
b) GEO?
c) a 2:1 lunar resonance orbit?
http://www.nss.org/settlement/ColoniesInSpace/colonies_chap08.html
In the'70s, Gerard O'Nielle posited an initial cost to build the first SPS(10 gw) from lunar resources would be about $100 billion, based upon the assumed launch costs for the space shuttle. 90 % of that cost was for the infra structure on the moon. Subsequent costs for each new 10gw SPS would then be about $10 billion each or one dollar/watt installed generating capacity. Since the fuel(sunlight) was free, the only other operating cost would be for maintenance of the SPS and we can't estimate that accurately because we don't know how much attrition of the solar cells would occur due to micro meteorite abrasion or radiation damage.
At ten cents/watt delivered cost for the electricity, the SPS would earn one billion dollars/year, enough to repay the initial investment in a mere decade. Most earth based generating(like hydro electric) are amortized over a presumed 30 to 40 year operating lifetime with a ten percent profit/year. I expect we could derive useful energy from an SPS for about a century, so there is a lot of profit potential at those presumed investment levels.