Linked in Rand's comment section:

"Orbiting Supply Depots
by Tom Hill
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An Orbiting Supply Depot

Imagine a place in Earth orbit where any craft in that orbit could stop and pick up hydrogen for use in rockets or fuel cells, water for its crew, and oxygen for multiple uses. For missions to LEO, these supplies could cut the need for frequent logistics flights from Earth's surface. For missions beyond LEO, these supplies could make such journeys feasible using the relatively modest-sized launchers that we have today.
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The Cost

I believe that this plan can be implemented for a cost on the order of $1B. This includes 52 delivery missions to orbit at $10M each, building the supply depot for $250M, launching the depot on a heavy EELV for $150M (a planning number - individual flights are negotiated), and building 52 delivery tanks for $100M. This is not a small amount of money, but given the payoffs already discussed, it is money much better spent than that thrown down a dead-end research project."
http://www.aiaa.org/tc/sos/communicator/jan_mar_2004/supply_depots.html