I’m a little confused about Profession. Looking for clarification.
Apprentice level makes sense. You can do the job and make 25gp per month.
LP and MP is where it gets a little confusing, because you have people working under you. You can earn 50gp as an LP and supervise 3 apprentices, increasing their productivity 50%.
What does this mean exactly? Do the apprentices also bring in gold? Since their productivity is increased, do they bring in more than the standard 25gp? Is this in addition to the 50gp the LP makes?
How much would a Master running a business with 2 journeymen (I assume these are the same as Licensed Practitioners?) and 4 apprentices working under him?

Sorry for the confusion!
Short Answer: The master of a fully-staffed business with 2 journeymen and 4 apprentices will personally earn twice his monthly wages, after expenses, each month.
Long Answer:
Let’s start with a smith, for example, where productivity is more clear cut.

  1. Apprentice, working alone, can manufacture 10gp per month. He earns, in turn, 10gp per month.
  2. Journeyman, working alone, can manufacture 20gp per month. He earns, in turn, 20gp per month.
  3. Master, working alone, can manufacture 40gp per month. He earns, in turn, 40gp per month.
    However, if a master employs 2 journeymen and 4 apprentices, production is as follows:
    4 apprentices - 10x1.5 = 15gp production each x4 = 60gp production
    2 journeymen - 20x1.5 = 30gp production each x2 = 60gp production
    1 master - 40gp production = 40gp production
    Total production = 160gp
    Total wages = 4x10 + 2x20 + 40 = 120gp
    Total productivity surplus = 40gp
    By supervising the journeymen and apprentices, the master increases production by 40gp. This is “added value”.
    This, then, explains why if try to hire a master armorer (under Specialists), it costs 75gp. This means that if a player hires a master armor (75gp), 2 journeymen (20gp each) and 4 apprentices (10gp each) the total cost 155gp, or just 5gp less than the production amount. The assumption is that the master armorer is capturing most of the value he creates!
    A moment’s reflection explains why this would be so. A master armorer can either choose to have his own shop or work on monthly retainer. If having his own shop were vastly more profitable, that’s what the armorer would do. Since there’s no “health insurance” or “employment contracts” in ACKS, there’s not a huge security benefit to salary work. It must therefore be about as lucrative to work on retainer, in order to make the choices economically equal. I used 5gp (3% of 160gp) as the difference. So a master armorer can make 75gp on retainer, or (160 in production - 80 in subordinate wages) 80gp from his own shop.
    For a fully-staffed professional firm, then, the master professional’s productivity is:
    4 x (25x1.5) = 150gp
    2 x (50x1.5) = 150gp
    1 x 100gp = 100gp
    = 400gp
    His total cost is 4x25 + 2x50 + 100 = 300gp.
    Therefore his surplus is 100gp. A professional with his own staff of 6 would therefore earn 200gp per month (100gp he pays himself + 100gp surplus). A professional on retainer with his own staff of 6 would probably earn 195gp or so.
    Note that the surplus is always equal to the full wages of the master, meaning a master who runs his own fully-staffed shop earns 2x his wages. You can use that rule of thumb to quickly calculate this for any professional.

Oh man, this is great stuff and makes perfect sense. Thanks for spelling it all out for me.

If a PC can swing more GP than their GP Threshold, do they earn XP from Profession and Crafting?

No, you don’t earn XP from profession and crafting.

I can see where one might be tempted to award xp for it, but I’d suggest that, if you’re using a predictable formula to determine income, you shouldn’t award xp for activitis that have no chance to fail.

You’re right, Charlatan. I was just making sure this wasn’t the case.