Continuously add to a parameter at each call

I am using xsl to manipulate data contained in a MS Project file and my
issue is with adding the total cost of certain tasks.  Currently I am
checking through all tasks to meet a certain condition and when there is
a match I want to obtain the cost of the task and set that value to a
parameter.  My desired end result should calculate the total cost of all
tasks that satisfy the condition.  Based on the code below, I calculate
the cost of a task if the first two "if" statements are satisfied.  I
want to have a parameter that I can update everytime I get the cost of a
task and keep a running total of the overall cost.

Can someone tell me what the code should be inside the template
"TotalCost" to keep a running total of the cost???

<xsl:if test="$task_uid = $temp_task_id"> <!-- Compare ID related to
Assignment -->

	<xsl:if test="$task_wbs = $temp_task_wbs">      <!-- Seperate
low-level tasks according to top-level -->

		<xsl:variable name="Cost"
select="*[local-name()='Cost']" />

            <xsl:call-template name="TotalCost">
 	           <xsl:with-param name="Tot_Cost" select="$Cost"/>
            </xsl:call-template>



I've been trying this for two days and cannot get a good result, so any
help is very much appreciated

Thankyou
Emil S.