XPath and calculations

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Ok, this might be a dumb question. I've got a secondary data source
with a structure like
....
<Season>
<Season_Id>1</Season_Id>
<Lodging>100</Lodging>
<Meals>25</Meals>
<Days_In_Season>10</Days_In_Season>
</Season>
<Season>
<Season_Id>2</Season_Id>
<Lodging>130</Lodging>
<Meals>35</Meals>
<Days_In_Season>5</Days_In_Season>
</Season>
.....

And I'd like to calculate the total cost over all seasons.
Something like sum(((Lodging + Meals) * Days_In_Season))
Any method of doing this with a XPath expression for use in a formula?