var list = new List<string> { "1", "124руб", "2", "424руб","3", "4", "5"};
var result = project.Lists["list"];
for (int i = 0; i < list.Count; i++){
result.Add(list[i]);
if ((i + 1) < list.Count && !list[i+1].Contains("руб") && !list[i].Contains("руб")) {
result.Add("не...