First, extract your numbers-ending-in-quote with a regex.
You can then use the Regex.Match overload that takes a MatchEvaluator as teh second parameter to feed each match individually to a function, which returns the new string.
Regex regex = new Regex(@"\d+(?=')");
string replaced = regex.Replace(inputString, new MatchEvaluator(ReplaceMatch));
The ReplaceMatch method would be along the lines of:
private string ReplaceMatch(Match m)
{
double feet = double.Parse(m.Value);
return string.Format("{0:N2}", feet * 0.3048);
}