SELECT 
  cscart_product_prices.product_id, 
  COALESCE(
    cscart_master_products_storefront_min_price.price, 
    MIN(
      IF(
        cscart_product_prices.percentage_discount = 0, 
        cscart_product_prices.price, 
        cscart_product_prices.price - (
          cscart_product_prices.price * cscart_product_prices.percentage_discount
        )/ 100
      )
    )
  ) AS price 
FROM 
  cscart_product_prices 
  LEFT JOIN cscart_master_products_storefront_min_price ON cscart_master_products_storefront_min_price.product_id = cscart_product_prices.product_id 
  AND cscart_master_products_storefront_min_price.storefront_id = 1 
WHERE 
  cscart_product_prices.product_id IN (
    278, 280, 282, 247, 248, 241, 244, 245, 
    246, 240, 6, 66, 166, 214, 219, 221, 223, 
    192, 194, 195, 196, 197, 198, 199, 200, 
    201, 202, 203, 204, 205, 206, 207, 208, 
    209, 210, 211, 212, 213, 215, 217, 218, 
    220, 222, 224, 225, 226, 227, 228, 229, 
    230, 231, 232, 233, 234, 235, 236, 237, 
    238, 239, 242, 243, 78, 79, 80, 81, 82, 
    83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 
    93, 94, 95, 96, 97, 100, 101, 102, 103, 
    104, 105, 106, 107, 108, 109, 110, 111, 
    112, 113, 114
  ) 
  AND cscart_product_prices.lower_limit = 1 
  AND cscart_product_prices.usergroup_id IN (0, 1) 
GROUP BY 
  cscart_product_prices.product_id

Query time 0.00116

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "254.57"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": true,
      "cost_info": {
        "sort_cost": "192.00"
      },
      "nested_loop": [
        {
          "table": {
            "table_name": "cscart_master_products_storefront_min_price",
            "access_type": "system",
            "possible_keys": [
              "PRIMARY"
            ],
            "rows_examined_per_scan": 0,
            "rows_produced_per_join": 1,
            "filtered": "0.00",
            "const_row_not_found": true,
            "cost_info": {
              "read_cost": "0.00",
              "eval_cost": "0.20",
              "prefix_cost": "0.00",
              "data_read_per_join": "16"
            },
            "used_columns": [
              "storefront_id",
              "product_id",
              "price"
            ]
          }
        },
        {
          "table": {
            "table_name": "cscart_product_prices",
            "access_type": "ALL",
            "possible_keys": [
              "usergroup",
              "product_id",
              "lower_limit",
              "usergroup_id"
            ],
            "rows_examined_per_scan": 296,
            "rows_produced_per_join": 191,
            "filtered": "64.86",
            "cost_info": {
              "read_cost": "24.17",
              "eval_cost": "38.40",
              "prefix_cost": "62.57",
              "data_read_per_join": "4K"
            },
            "used_columns": [
              "product_id",
              "price",
              "percentage_discount",
              "lower_limit",
              "usergroup_id"
            ],
            "attached_condition": "((`danishecarter_31jan`.`cscart_product_prices`.`lower_limit` = 1) and (`danishecarter_31jan`.`cscart_product_prices`.`product_id` in (278,280,282,247,248,241,244,245,246,240,6,66,166,214,219,221,223,192,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,215,217,218,220,222,224,225,226,227,228,229,230,231,232,233,234,235,236,237,238,239,242,243,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114)) and (`danishecarter_31jan`.`cscart_product_prices`.`usergroup_id` in (0,1)))"
          }
        }
      ]
    }
  }
}