You can use the Replace
function of the string class like so:
string str = "25,000";
str = str.Replace(",", "");
EDIT:
As suggested by Matthew Watson
from the comments
If the string is returned from string.Format("{0:n}", 999999) run on a machine in a locale that uses "." as the thousands separator, this will fail
updated answer:
string num = "25,000";
NumberFormatInfo currentInfo = CultureInfo.CurrentCulture.NumberFormat;
num = num.Replace(currentInfo.NumberGroupSeparator, "");