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 (
    6303, 6304, 6305, 6306, 1500, 1501, 1502, 
    1503, 3726, 5206, 5207, 5208, 5209, 
    6736, 1490, 1491, 1492, 1493, 3724, 
    5198, 5199, 5200, 5201, 6735, 1495, 
    1496, 1497, 1498, 3164, 3725, 5202, 
    5203, 5204, 5205, 1485, 1486, 1487, 
    1488, 3163, 3723, 5194, 5195, 5196, 
    5197, 1429, 1430, 1431, 1432
  ) 
  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 0.00139

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 (6303,6304,6305,6306,1500,1501,1502,1503,3726,5206,5207,5208,5209,6736,1490,1491,1492,1493,3724,5198,5199,5200,5201,6735,1495,1496,1497,1498,3164,3725,5202,5203,5204,5205,1485,1486,1487,1488,3163,3723,5194,5195,5196,5197,1429,1430,1431,1432))",
        "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)))"
      }
    }
  }
}