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 (8, 9) 
  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 0.00058

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "26.19"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": false,
      "table": {
        "table_name": "cscart_product_prices",
        "access_type": "ALL",
        "rows_examined_per_scan": 254,
        "rows_produced_per_join": 1,
        "filtered": "0.40",
        "cost_info": {
          "read_cost": "26.09",
          "eval_cost": "0.10",
          "prefix_cost": "26.19",
          "data_read_per_join": "24"
        },
        "used_columns": [
          "product_id",
          "price",
          "percentage_discount",
          "lower_limit",
          "usergroup_id"
        ],
        "attached_condition": "((`mobikul_delivery_boy_livedemo`.`cscart_product_prices`.`lower_limit` = 1) and (`mobikul_delivery_boy_livedemo`.`cscart_product_prices`.`product_id` in (8,9)) and (`mobikul_delivery_boy_livedemo`.`cscart_product_prices`.`usergroup_id` in (0,1)))"
      }
    }
  }
}