HMM well economies are always hard, unfortunately. I can say with confidence the most one person can ever earn per day is 100 creds - it's random whether or not you earn any per post, but that's the max. As a frame of reference, despite being the co-owner of the site, I have ... 340c to my name? So I guess base on how much time you value from someone - if you assume it takes anywhere from like 20-200 posts (probably with an average in the middle) per day in order to get your full 100, how much time on the site is your stuff worth? x'D
tl;dr economies are hard, and the value will change further once we open the site itself since after the initial WE CAN BUY THINGS the amount of creds in the system will likely go down so hopefully they'll remain relatively scarce.
... ... ... imo, I think 1-10 creds is a reasonable tip amount, and I personally probably wouldn't charge over 100 for things since that's a good amount of posting (a whole day's earnings, so to speak). Does that help?
//rambles