You can solve this by adding items in an array. Use array index for calculating price of an item. Out put of your program will be like this:
Enter item number
1. Eggs
2. Bread
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string[,] items = new string[,] {{"Eggs","10"}, {"Bread","15"}};
string input;
int price=0;
int quantity;
int index;
do {
input = Console.ReadLine();
Console.WriteLine("Enter quantity: ");
index = Convert.ToInt32(input)+1;
quantity = Convert.ToInt32(Console.ReadLine());
price += quantity * Convert.ToInt32(items[index,1]); // accessing price of item
}while(input != "S");
Console.WriteLine("Your total: "+price);