I got to wondering a while back about how the karma works too, Azayzel, and you're not too far off (I had to ask Alexander to get the scoop). It's figured based entirely on the karma giver's status (I had thought it might have something to do with the relative statuses of the giver and the givee, but nope).
Each person seems to start out with 1 point of influence (after they've posted at least once), then it follows the formula. Here's the formula that I worked out from what Alexander told me, it seems to hold true from what I've seen:
(# of days registered/30) + (# of posts/10) + (Giver's present Karma/40)
Remember, these are all based on the karma
giver's stats.
So, for example if a person who had been registered 300 days, had posted 600 items, and had a present Karma of 80 were to give someone else Karma:
(300/30) + (600/10) + (80/40)
Or:
10 + 60 + 2 = 72
So If this theoretical person were to tap someone who had, say 28 karma points already, the tapee's karma would increase to 100 points. And yes, I picked nice, tidy numbers to make it easy on myself -- I'm used to the computer doing my arithmetic for me (and most of my spelling, too, for that matter).
The biggest single factor is how much you post, the more you post the faster your givable karma goes up.