My guild usually has about 20 or more people log on and off per day. Most don't play too much but you can say they all play about 2 hours. Thats 280 hours of gamplay amongst my guild. This number can be bumped up a little on weekends, so lets say about 320 hours of gameplay for my guild.

Given that blue food requires three ingredients and assuming 3/3 Chef - myself, my guild provisioners and any guildie that is cooking for themselves must scrounge up 240 ingredients per week.

We cant loot cities anymore without serious repercussions. You wont find additives out in the world like we find alchemy plants or cloth/wood/metal. In fact no one will. The kiosks which are full of stuff people looted from the cities will be almost barren, only tiny little lots of stuff looted from bandit delves and villages overrun by bad guys. God only knows how expensive these will climb. Lets just say 75g each for example (the price in kiosks of hard to get ingredients such as Bear Haunch)

My guild may need to spend upwards of 18,000g per week to stay fed. Am I really going to have to go around and collect lunch money from all my guildies each week?

75g is actually a optimistic price for additives. You might run a dungeon and come across three flour, one seasoning, two cheese. Tiny amounts. Who the heck is going to keep these tiny amounts in their bag/bank unless they are sure they can get serious profit on them. Fruits and Vegetables can be bought from a Grocer for 150g each. Hard to get meat will likely hit at least that price (Red Meat which can only be had by killing a cow someone owns, Poultry which also runs a bounty risk) and why should anybody sell an additive so low knowing that it is absolutely necessary for any green food above level15 and any blue food above level 20.

The possible economic impact to guilds is so outrageous that I have a very simple prediction - Provisioning will come to an end except to hardcore PvP and Raiding guilds which can easily generate the gold. Humble casual PvX guilds will simply have to starve.