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 (
    6736, 613, 617, 3121, 3119, 5039, 5038, 
    5037, 5036, 5406, 6679, 5407, 5408, 
    4041, 6974, 7819, 7181, 7179, 1195, 
    1197, 1196, 6135, 6134, 6133
  ) 
  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 0.00043

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "21.31"
    },
    "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": 34,
        "rows_produced_per_join": 6,
        "filtered": "19.12",
        "index_condition": "(`dbggbern`.`cscart_product_prices`.`product_id` in (6736,613,617,3121,3119,5039,5038,5037,5036,5406,6679,5407,5408,4041,6974,7819,7181,7179,1195,1197,1196,6135,6134,6133))",
        "cost_info": {
          "read_cost": "20.66",
          "eval_cost": "0.65",
          "prefix_cost": "21.31",
          "data_read_per_join": "156"
        },
        "used_columns": [
          "product_id",
          "price",
          "percentage_discount",
          "lower_limit",
          "usergroup_id"
        ],
        "attached_condition": "((`dbggbern`.`cscart_product_prices`.`lower_limit` = 1) and (`dbggbern`.`cscart_product_prices`.`usergroup_id` in (0,1)))"
      }
    }
  }
}