Hmm, well first off you would need to base it off the kapi players to make it link to the game. Idealy players would post their daily profits but since thats not gona happen the best way I could see is a total worth system.
Basicialy each company is issued with 1000 shares (just a easy number) that reflect their FA + a moderated cash. Just a suggestion but FA + half of cash is a fair number, keeps cash rich bizs from pushing their stocks around ect. SO assume one company is worth 1b so each share should be $1.00. A 4.56b co. would be 1000 shares at $4.56.
Thats the begining now you just have to sellect a 100 companies or so as a baisis. Many more and it gets over-complicated to deal with. Then assign each new person to join up x number of $ to invest as they feel fit with the objective to own the max amount of compant value.
Prices change based on market forces, informed by the 'real' change in company value within kapi. So for example that 1b co made on avreage 100m a day asset growth (unralistic I know) so its stock price will rise by 10% but if another co is rising by 200m a day (from 1b) its company value will be higher than 20% extra as its expected to countine its higher profits so the price will reflect the expecttation of the market.
The dificualt part is gona be keeping people honest, you would have to keep fairly undeniable records of each transation between players or deal with it as a centeral market with each sale beaing made only with a trusted player in the middle. IE jack puts up 10 shares at $2 ea to the market, MR Honest (mod) then posts the sale price/amt to the forum and then Jill can contact Mr Honest and her running account is debited 20c and the shares moved to her. Sounds simple but once you start dealing with multiple people all wanteing to trade as fast as is posisiable things will invariably get confused. If you start somthing its gona have to be air tight or risk the whole think going up in smoke!
Thats how I would work it anyway

If you do start somthing (or want to

) give me a call
H