By comparison, one of the longest DC power lines in existence is the pacific intertie:
http://en.wikipedia.org/wiki/Pacific_DC_IntertieWhich says it carries 3 gigawatts maximum.
It's also built overland, whereas a world transit system would need to be mostly (and much more expensiuvely) underwater. But I'll completely ignore that.
World energy consumption is 21~ terawatts. Let's say 1/3 needs to be used at night. That's 7 terawatts = 2,000x the most ambitious existing lines' capacity. Add on quite a bit to that for additional average losses per mile for higher distances, probably more like 3,000+ (being
VERY generous and futuristic here)
Now add on the extra length (so far we've been talkign about cross-sectional capacity costs, not how much longer it would be). It needs to ring the entire world, either 2 or 3 times, or go up and down each continent chain as well. Something in the neighborhood of 50,000 miles, let's say. That's 50x longer than the intertie, so we're up to 150,000x the cost.
I can't seem to find a cost for the intertie's initial construction, but some recent maintenance and upgrades to it were estimated at $428 million. So just for upgrade/maintenance of your cable, as a scaled comparison, you'd be looking at 428,000,000 * 150,000 =
$64 trillion with today's technology...