Wanting the sum of 1 field

Hi

I'm new to this, so just looking for some help.

I have the code as below, but I would like to know if I can get the sum of the single field rather than all fields.
So then I have a field called "ChargeTotal" and I would like to get the sum of the totals in that field.
EG - if the 4 lines in the database are 10 / 20 / 20 / 10, I would like to return a single value of 60 for that 1 field.

Is this possible and where / what do I insert to get the sum of those values.

<soap12:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap12="http://www.w3.org/2003/05/soap-envelope">
<soap12:Body>
<GetCharges xmlns="http://www.registernow.com.au/secure/Services/SecureServices.asmx">
<eventId>1234</eventId>
<ApiKey>test1234</ApiKey>
</GetCharges>
</soap12:Body>
</soap12:Envelope>

Thanks in Advance.