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 (
    242, 243, 6, 66, 166, 214, 219, 221, 223, 
    240, 241, 244, 245, 246, 248, 247, 278, 
    280, 282
  ) 
  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 0.00077

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "91.98"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": true,
      "cost_info": {
        "sort_cost": "38.00"
      },
      "table": {
        "table_name": "cscart_product_prices",
        "access_type": "ALL",
        "possible_keys": [
          "usergroup",
          "product_id",
          "lower_limit",
          "usergroup_id"
        ],
        "rows_examined_per_scan": 254,
        "rows_produced_per_join": 38,
        "filtered": "14.96",
        "cost_info": {
          "read_cost": "46.38",
          "eval_cost": "7.60",
          "prefix_cost": "53.98",
          "data_read_per_join": "912"
        },
        "used_columns": [
          "product_id",
          "price",
          "percentage_discount",
          "lower_limit",
          "usergroup_id"
        ],
        "attached_condition": "((`pankajecarter_systemtwo`.`cscart_product_prices`.`lower_limit` = 1) and (`pankajecarter_systemtwo`.`cscart_product_prices`.`product_id` in (242,243,6,66,166,214,219,221,223,240,241,244,245,246,248,247,278,280,282)) and (`pankajecarter_systemtwo`.`cscart_product_prices`.`usergroup_id` in (0,1)))"
      }
    }
  }
}