pricing dilemna

I am part of a team that is making a virtual trading game. We usually use US stock market feeds to fix the price of our equity stocks. What we did this year was putting in a local effect ( within the sample space of players playing the game ) depending upon the volumes traded. And we are having a tough time doing that. Could anyone actually quantify the buy/sell sentiment and come up with some sort of formula that prices the equity stock. The final price is going to be a mix of both the actual nasday/nyse price and the local effect. I need help !!! thanks