Since it looks like we're going with the,

[quote]Option B: Winning contests/challenges and beating missions award Credits, which can be used in a shop system to buy Armory upgrades, Ability upgrades, and subplots.  
TENTATIVE: 4 for Armory, 6 for Ability, 10 for subplot 
Suggestion: Nerf mission credits to half or double credits needed with every purchase :P
[/quote]

I have a couple of thoughts. Mission rewards are in Credits and Lien (RWBY's Currency?), so why don't we use both of those currencies? Keep in mind these are just my thoughts on the subject. GM has the final word on the stuff.

Armory/Tech/Weapon upgrades/items would use Lien, lore wise, the Lien would cover the costs of materials of said upgrade/item. Small upgrades would be like, a couple thousand maybe, since the mission rewards are in the 1,000 - 4,000 range already. Medium upgrades would be somewhere around 10,000 and maybe large upgrades would be 20,000. 
Small upgrades being like, a new attachment to an existing weapon, or a small piece of gear like a grappling hook.
Medium upgrades being an actual upgrade of said items, like a direct upgrade in killing power or a new function(?)
Heavy upgrades would be a complete overhaul of an existing thing, or maybe a completely new weapon.

We also have to keep in mind our own characters' abilities, concerning these Armory/Tech/Weapon upgrades. My character, Ineko, is pretty darn good at electronics, armor, etc, but he probably can't make a good sword to save his life. If he wanted to make a sword, it might be more expensive than someone else's attempt, who is good at making swords.

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Since 'credits' are a reward too, at first I thought these were like college credits, but apparently not(?), these could be used to purchase Semblance/Ability upgrades. As well as subplots, as said in the quote/general Gdoc. I'm not sure how to apply credits to ability upgrades. We could use the Lien thing, outlined above, with the Light, Medium, and Heavy upgrades, each one being more expensive than the last. Mission rewards so far are in the 1 to 5 Credit rewards. It seems like we've all followed a similar system, each grade up is adding a credit.

[i]
D - 1 CR
C - 2 CR
B - 3 CR
A - 4 CR
S - 5 CR
SS - 6 CR[/i]

Which is cool because it's at least consistent. Maybe we can price it accordingly?
[i]Light Ability/Semblance Upgrade  - 5 to 6 CR
Medium Ability/Semblance Upgrade - 10 to 12 CR
Heavy Ability/Semblance Upgrade  - 20 to 24 CR[/i]
These are so heavily priced because they're Ability/Semblance upgrades, stuff that borders the supernatural, I guess. Upgrades shouldn't come all that often. Then again, I'm not exactly sure. We should also keep higher ranked missions no more than the lower ranking missions, so that we don't power level speed run MLG pro our way through several SS missions and get that Heavy Upgrade early.

As for the subplot thing, I honestly have no idea how to price it, maybe we don't. Since paying CR for a subplot would mean giving up a potential upgrade to your Semblance. Some people might not want to give up that upgrade.

As for exact prices, I dunno. Maybe PM the upgrade in question to Lugu the GM, or a Co-GM, and they'll discuss it and come up with an exact price.