Dont ask me .. i dont know
When CRM 2015 is released calculated fields will be part of the product so that might help
http://www.powerobjects.com/blog/2014/11/17/calculated-fields-in-dynamics-crm-2015/
Otherwise you'd need to use plugins to calculate the value when the entity is created, updated, modified or deleted and then in the same step, update the entity where the total is to be displayed.
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