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 (
    1856, 1918, 1919, 1862, 1861, 1857, 1883, 
    1860, 1859, 1854, 1923, 1858
  ) 
  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 0.00095

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": 14,
      "filtered": 2.448978901,
      "index_condition": "cscart_product_prices.product_id in (1856,1918,1919,1862,1861,1857,1883,1860,1859,1854,1923,1858)",
      "attached_condition": "cscart_product_prices.lower_limit = 1 and cscart_product_prices.usergroup_id in (0,1)"
    }
  }
}

Result

product_id price
1854 46000.00000000
1856 50000.00000000
1857 50000.00000000
1858 23500.00000000
1859 23500.00000000
1860 23500.00000000
1861 23500.00000000
1862 23500.00000000
1883 30000.00000000
1918 13000.00000000
1919 23500.00000000
1923 46000.00000000