How to use display "mask" in C#

Here's what I'm doing to take a "float" number of, say 1259.9500 to 1259.95
      lblTotalDonations.Text = dsDonationTotals.DonationTotals[0].DonationTotal.ToString().Substring(0, dsDonationTotals.DonationTotals[0].DonationTotal.ToString().Length - 2);

But what I really want is to apply a "mask" and take the Total (1259.9500)  to $1,259.95
Is there a "money" mask? How do you do this? (From Codebehind)

Using C# (Visual Studio Pro 2008) Web appStart Free Trial