Ignoring energy, that's 36.2k mass to generate 104 mass which takes 348s (just short of 6 mins) to pay back. (In comparison, a ringed T3 mex is 5400 mass which takes 200s to pay for itself, but you wouldn't expect to beat that.)
If you did the same with T3 fabs (6.5, yeah...) the payback time is 398s but you get about 1500 excess power (maybe more if I remembered adjacencies incorrectly). Still less efficient with a spare T3 pgen thrown in.
But don't forget that it'll take a while to build, especially with engineers bumping around each other to get out the way all the time.
Otherwise, this much mass gets you half a T3 arty or a Megalith or Soulripper or 17 strats.