Please see details on Purcahse Field Mapping: http://forums.adobe.com/docs/DOC-1632
Under "Order Form - Choose Multiple Items From a List of Items" example, you can see how a predefined/fixed amount can be associated with options.
If you are asking if the form can compute total amount in purchase fields or how the fields can be formatted to do so, then we currently do not support this.
Hope this helps,
Thank you...that helped! I appreciate the info.