Getting to 30km height ideally costs just the potential energy and electric lifts are pretty close to achieving that, so it would be desirable to get out of thick atmosphere that way.

Is it possible with today's materials to build a blimp that can just float for years without needing a full replacement? I think it could be compartmentalized. If the blimp has individual chambers such that a sever damage in one point can be compensated by pumping gas to a different chamber while fixing that spot, while smaller damage gets repaired by autonomous robots, while using at least one redundant tether so you can maintenance or replace these, too, ... you could have the whole structure maintainable. Then it comes down to leakage cost and longevity of the materials.

If you have 50 launches per year, you get a life time budget for construction and maintenance of $1.35B/year. 

 Of course, with two launches per day, that budget increases to almost $20B/year etc.