Good estimate of total GP?

As others have stated here, this individual sounds like a lost cause to me. A tremendous amount of informative tutorials and guides exist to help players develop their rosters for pretty much every aspect of the game. If someone chooses to ignore that curated advice and refuses to alter their development strategy then there is precious little that can be done.

If a guild insists on keeping this person within their ranks they are only "shooting themselves in the foot" which can cause a negative effect on the other members. I totally understand loyalty and wanting to build a guild but sometimes the cost isn't worth the payout.