try this
dt.AsEnumerable.Sum(Function (x) If(x(1).ToString.Replace(" ","").IsNumeric, CDbl(x(1).ToString.Replace(" ", "")), 0 ))
try this
dt.AsEnumerable.Sum(Function (x) If(x(1).ToString.Replace(" ","").IsNumeric, CDbl(x(1).ToString.Replace(" ", "")), 0 ))