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 (
    4, 1, 5, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 
    17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 
    27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 
    37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 
    47, 48, 49, 50, 51
  ) 
  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 0.00102

JSON explain

{
  "query_block": {
    "select_id": 1,
    "filesort": {
      "sort_key": "cscart_product_prices.product_id",
      "temporary_table": {
        "table": {
          "table_name": "cscart_product_prices",
          "access_type": "ALL",
          "possible_keys": [
            "usergroup",
            "product_id",
            "lower_limit",
            "usergroup_id"
          ],
          "rows": 253,
          "filtered": 13.99961281,
          "attached_condition": "cscart_product_prices.lower_limit = 1 and cscart_product_prices.product_id in (4,1,5,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51) and cscart_product_prices.usergroup_id in (0,1)"
        }
      }
    }
  }
}