var query1 = from sp in db.SanPhams
            group sp by sp.MaLoai into LspGroup
            select new
            {
                MaLoai = LspGroup.Key,
                TongTien = LspGroup.Sum(t => t.DonGia * t.SoLuong)
            };

var query2 = from t in query1
             join lsp in db.LoaiSanPhams on t.MaLoai equals lsp.MaLoai
             select new
             {
                 MaLoai = lsp.MaLoai,
                 TenLoai = lsp.TenLoai,
                 TongTien = t.TongTien
             };

dgvLoaiSanPham.ItemsSource = query2.ToList();