The excess stock value is simply the sum of all your items excess quantity X the cost. So if this looks incorrect there are a few possibilities:
Firstly, go to the order plan, select excess stock, and sort by the order quantity column so that the items with the most excess are at the top. Then check them for the following:
1. Is the cost data correct? Occasionally items may have had an incorrectly large cost associated with them.
2. Are the on hand and on order amounts correct? If there are large amounts of on hand or on order values this will contribute to a large excess stock value
3. Has all the sales history been imported correctly? If not all sales history has been imported, then the forecasts will be very low, meaning that the system is flagging your current stock level as higher than you need and therefore in excess.