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 (
    275133, 275759, 275765, 276261, 276352, 
    276420, 276421, 276426, 276427, 276428, 
    276429, 276430, 276431, 276432, 276433, 
    276434, 276435, 276436, 276437, 276438
  ) 
  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 0.00102

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "14.01"
    },
    "grouping_operation": {
      "using_filesort": false,
      "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": "0.10",
              "prefix_cost": "0.00",
              "data_read_per_join": "16"
            },
            "used_columns": [
              "storefront_id",
              "product_id",
              "price"
            ]
          }
        },
        {
          "table": {
            "table_name": "cscart_product_prices",
            "access_type": "range",
            "possible_keys": [
              "usergroup",
              "product_id",
              "lower_limit",
              "usergroup_id"
            ],
            "key": "product_id",
            "used_key_parts": [
              "product_id"
            ],
            "key_length": "3",
            "rows_examined_per_scan": 20,
            "rows_produced_per_join": 3,
            "filtered": "20.00",
            "index_condition": "(`s2cart`.`cscart_product_prices`.`product_id` in (275133,275759,275765,276261,276352,276420,276421,276426,276427,276428,276429,276430,276431,276432,276433,276434,276435,276436,276437,276438))",
            "cost_info": {
              "read_cost": "13.61",
              "eval_cost": "0.40",
              "prefix_cost": "14.01",
              "data_read_per_join": "95"
            },
            "used_columns": [
              "product_id",
              "price",
              "percentage_discount",
              "lower_limit",
              "usergroup_id"
            ],
            "attached_condition": "((`s2cart`.`cscart_product_prices`.`lower_limit` = 1) and (`s2cart`.`cscart_product_prices`.`usergroup_id` in (0,1)))"
          }
        }
      ]
    }
  }
}

Result

product_id price
275133 0.00000000
275759 0.00000000
275765 0.00000000
276261 0.00000000
276352 0.00000000
276420 0.00000000
276421 0.00000000
276426 0.00000000
276427 0.00000000
276428 0.00000000
276429 0.00000000
276430 0.00000000
276431 0.00000000
276432 0.00000000
276433 0.00000000
276434 0.00000000
276435 0.00000000
276436 0.00000000
276437 0.00000000
276438 0.00000000