25 followers with the "treasure hunter" trait

[Kantoro]

3-5 TH is enough, making money off garrison missions is really about quantity rather than quality. You can actually make quite a bit of money with the shipyard, but don't tell anyone, or Blizzard will nerf it :P

[schwarzkopf]

If you only care about gold missions, go for it.
If you want any value at all, mix them with scavengers.

I'm already overflowing with GRs and Oil, and Apexis crystals - I have 6 or so THs and rarely ever get them all used on any significant gold mission (>50g base).

It isn't as complicated as people make out to get your followers to a point where they make close to the top gold a week, mainly because gold missions aren't as common on average as some people make out.