[acidburnNSA]

Indeed, which is why it's not done commercially today. Note also that total fuel cost is about 5% of the cost of a nuclear plant, and that includes mining, milling, enrichment, and fabrication. As seawater extraction becomes cheaper and uranium mines run low (no time soon), it will basically be a wash economically to switch to the renewable uranium. Also, breeder reactors don't need you to enrich the fuel, so that counterbalances any increased extraction cost. On the downside most (but not all) breeders require chemical reprocessing which so far has been very expensive. With development this too could go down.

The key point is that we would never run out of fuel as a species if we went big with nuclear.