Great question, and one that I don't have a quantitative reply to on hand. The thing with uranium is that it is so unimaginably energy dense that with the success we've seen so far in seawater extraction technology, it's hard to imagine the energy required for extraction being anything but minuscule in comparison. Obviously there's a limit of concentration where this becomes not true but my understanding is that we are not near that limit.

Here's a summary of some recent work on seawater uranium extraction: https://www.forbes.com/sites/jamesconca/2016/07/01/uranium-s...

And here's a special edition of Industrial and Chemical Engineering dedicated to the topic: http://pubs.acs.org/toc/iecred/55/15#UraniuminSeawater