Drupal Rules, create the SUM of a field decimal values
You could enable PHP filter module (in core package), what will expose a 'run PHP' rule. There you could run some queries. Have a look at db_select and db_query for how to construct them (there are good advices in the comments as well.). 

Proceed with caution though as you are communicating directly against the database. Sanitize the in- and outputs.

If it is only for display purposes, you could achieve a similar effect with Views and aggregation enabled. Enable aggregation in the advanced section. Add the final_value field, and select SUM. This is a much safer approach.

UPDATE

I suppose you are triggering the rule on the save event of the Order-node? In that case the Order is in the $node variable and you could go as follows;

$node-&gt;field_name = $calculated_value;
node_save($node);