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 (
    10521, 10520, 10519, 10518, 10517, 10516, 
    10515, 10514, 10513, 10512, 10511, 
    10510, 10509, 10508, 10507, 10506, 
    10505, 10504, 10503, 10502, 10501, 
    10500, 10499, 10498, 10497, 9839, 12702, 
    9841, 9848, 9840, 9844, 9843
  ) 
  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 0.00070

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_prices",
      "access_type": "range",
      "possible_keys": ["usergroup", "product_id", "lower_limit", "usergroup_id"],
      "key": "product_id",
      "key_length": "3",
      "used_key_parts": ["product_id"],
      "rows": 32,
      "filtered": 99.23484039,
      "index_condition": "cscart_product_prices.product_id in (10521,10520,10519,10518,10517,10516,10515,10514,10513,10512,10511,10510,10509,10508,10507,10506,10505,10504,10503,10502,10501,10500,10499,10498,10497,9839,12702,9841,9848,9840,9844,9843)",
      "attached_condition": "cscart_product_prices.lower_limit = 1 and cscart_product_prices.usergroup_id in (0,1)"
    }
  }
}

Result

product_id price
9839 2150.58000000
9840 2150.58000000
9841 2150.58000000
9843 2150.58000000
9844 2150.58000000
9848 2150.58000000
10497 3599.16000000
10498 3599.16000000
10499 3599.16000000
10500 3599.16000000
10501 3599.16000000
10502 4033.73000000
10503 4033.73000000
10504 4033.73000000
10505 4033.73000000
10506 4033.73000000
10507 4033.73000000
10508 4033.73000000
10509 4189.73000000
10510 4189.73000000
10511 4189.73000000
10512 4189.73000000
10513 4189.73000000
10514 4189.73000000
10515 4189.73000000
10516 4189.73000000
10517 4189.73000000
10518 3365.16000000
10519 3365.16000000
10520 3365.16000000
10521 3365.16000000
12702 2150.58000000