--== SAMPLE DATA ==--
DECLARE @XML XML = '<YourXML><TheData value="3.100000000000e+003" /></YourXML>';
--== VIEW SAMPLE DATA==--
SELECT @XML;
/* Looks like this: -
<YourXML>
  <TheData value="3.100000000000e+003" />
</YourXML>
*/
--==Convert from scientific notation==--
SELECT Data.value('(@value)[1]', 'REAL')
FROM @XML.nodes('./YourXML/TheData') yourXMLData(Data);