SELECT 
  cscart_product_prices.product_id, 
  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 
WHERE 
  cscart_product_prices.product_id IN (
    6013, 6014, 6015, 6016, 6017, 6018, 6019, 
    6020, 6196, 6197, 6198, 6199, 6200, 
    6732, 2836, 2837, 2838, 2839, 2840, 
    6422, 6423, 6424, 6425, 2937, 2938, 
    2939, 2940, 2941, 6522, 6523, 6524, 
    6525, 2189, 2190, 2191, 2192, 2193, 
    2194, 2195, 2196, 2197, 2720, 2721, 
    2722, 2723, 2769, 2770, 2771
  ) 
  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 0.00095

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "33.61"
    },
    "grouping_operation": {
      "using_filesort": false,
      "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": 48,
        "rows_produced_per_join": 9,
        "filtered": "20.00",
        "index_condition": "(`gaseus`.`cscart_product_prices`.`product_id` in (6013,6014,6015,6016,6017,6018,6019,6020,6196,6197,6198,6199,6200,6732,2836,2837,2838,2839,2840,6422,6423,6424,6425,2937,2938,2939,2940,2941,6522,6523,6524,6525,2189,2190,2191,2192,2193,2194,2195,2196,2197,2720,2721,2722,2723,2769,2770,2771))",
        "cost_info": {
          "read_cost": "32.65",
          "eval_cost": "0.96",
          "prefix_cost": "33.61",
          "data_read_per_join": "230"
        },
        "used_columns": [
          "product_id",
          "price",
          "percentage_discount",
          "lower_limit",
          "usergroup_id"
        ],
        "attached_condition": "((`gaseus`.`cscart_product_prices`.`lower_limit` = 1) and (`gaseus`.`cscart_product_prices`.`usergroup_id` in (0,1)))"
      }
    }
  }
}