C # static method with ref parameter - good idea?

I recently reorganized some code and now have a static utility class with this way:

const int x = 1;
public static string doWork(ref DataTable dt)
{
    decimal total = 0;
    foreach (DataRow row in dt.Select("COST_ID = " + x))
    {
        decimal annual = decimal.Parse(row["Cost"].ToString());
        total += decimal.Round(annual, 2);
    }
    return String.Format("{0:C}", total);
}

I simplified the example for clarity ...

Perhaps I experienced any negative consequences of this and placing the doWork method call in the code behind the ASP.NET application that affects many users? Does anyone know or have a link where I can read how, in terms of performance, the static method will work? Is it becoming a bottleneck of any kind?

EDIT:

Yes, I apologize, it was not a very good example, so let's say something else like this:

const int x = 1;
public static string doWork(ref DataTable dt)
{
    foreach (DataRow row in dt.Select("COST_ID = " + x))
    {
        row["Cost"] = 0.0;
    }
}

You say that A) I don’t even need a link here, since Datatable is already passed ref and B). Performance is not at all hit by “forwarding” all calls to one static method.