operation of the base probably does not fit into this budget. or at least not for long.
lots of possibilities for unforeseen complications here, hard to know how far you'll actually get on a given budget. this might be entirely optimistic.
It's been a few years since I read about it, but I think that was the estimate for both developing the tech and deploying it. Of course there's the chance that they spend all $50 billion and still fail to find a way to make carbon nanotubes work at macroscopic scale, but I bet they'd have learned quite a lot of other interesting things doing that research.
And they did have a lot of trouble deploying tethers from the space shuttle that one time.