Green Lantern would be fairly easy to do on a TV budget these days. The actual visual effects can be done fairly cheaply with todays technology. You would ditch the CGI costume from the movie for a practicle one and that right there is a huge savings in time and money. The series would be mostly set on Earth with a few sets for Oa. Villian/Alien of the week would be the biggest production cost but no more so than say Defiance will rack up.
The 60's Batman had very good production values for its time, much more so than the 70's Spiderman had.