LinqObj76.cs

static void LinqObj76()
{
    var goods = ReadItems<Item>("goods.txt");
    var prices = ReadItems<Cost>("prices.txt");

    var result = goods
        .GroupJoin(prices, g => g.Article, p => p.ItemArticle,
            (g, p) => p.DefaultIfEmpty().Select(y => new {g.Country, y?.ShopName, y?.Price}))
        .SelectMany(x => x)
        .GroupBy(x => x.Country)
        .Select(x => new
        {
            Country = x.Key,
            Count = x.Where(y => !string.IsNullOrEmpty(y.ShopName))
                .Select(y => y.ShopName)
                .Distinct()
                .Count(),
            MinPrice = x.Min(y => y.Price) ?? 0
        });

    var resultQuery =
        from g in goods
        join p in prices
            on g.Article equals p.ItemArticle
            into pMatch
        from p in pMatch.DefaultIfEmpty()
        select new {g.Country, p?.ShopName, p?.Price}
        into seq
        group seq by seq.Country
        into countryGroup
        select new
        {
            Country = countryGroup.Key,
            Count = countryGroup
                .Where(y => !string.IsNullOrEmpty(y.ShopName))
                .Select(y => y.ShopName)
                .Distinct()
                .Count(),
            MinPrice = countryGroup.Min(y => y.Price) ?? 0
        };

    Console.WriteLine(string.Join("\n", result));
    Console.WriteLine();
    Console.WriteLine(string.Join("\n", resultQuery));
}