first check what is the returen type of
db.selectquery_One(strSQL);
also try to put a brekpoint at
string totalExp = db.selectquery_One(strSQL).ToString();
and
string totalDep = db.selectquery_One(strSQL).ToString();
to see what value you are getting and as told by
Jibesh use
Double.TryParse
or
Double.Parse
to convert your value.