Building a stock market simulation

I am building a fantasy stock market simulation site. Things i need built are - user logins, unique portfolio pages per user - the ability for users to purchase and sell stocks (fake money) and have have their aggregate purchases appear in their portfolio, with a balance that fluctuates according to their trades - prices of stocks will need to fluctuate based on volume and also daily bonuses administered by me - all trades and user data should be stored in back end sql databases - standings pages, sortable by several categories - user discussion forums * i have already created the user logins, unique ports, and the ability to buy stocks. i am hung up getting the prices to fluctuate as a function of volume. You can use what i have already done or you can start from scratch, whichever is easier.