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 (
    1818, 1819, 3744, 3745, 3746, 3747, 5461, 
    5462, 5463, 5464, 5465, 5466, 5467, 
    5468, 5469, 5470, 5471, 5472, 5473, 
    5474, 5475, 5476, 5477, 5478, 5479, 
    5480, 5481, 5482, 5483, 5484, 5485, 
    5486, 5487, 5488, 5489, 5490, 5491, 
    5492, 5493, 5494, 5495, 5496, 5497, 
    5498, 5499, 5500, 5501, 5502
  ) 
  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 0.00136

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "33.61"
    },
    "grouping_operation": {
      "using_filesort": false,
      "table": {
        "table_name": "cscart_product_prices",
        "access_type": "range",
        "possible_keys": [
          "usergroup",
          "product_id",
          "lower_limit",
          "usergroup_id"
        ],
        "key": "product_id",
        "used_key_parts": [
          "product_id"
        ],
        "key_length": "3",
        "rows_examined_per_scan": 48,
        "rows_produced_per_join": 9,
        "filtered": "20.00",
        "index_condition": "(`gaseus`.`cscart_product_prices`.`product_id` in (1818,1819,3744,3745,3746,3747,5461,5462,5463,5464,5465,5466,5467,5468,5469,5470,5471,5472,5473,5474,5475,5476,5477,5478,5479,5480,5481,5482,5483,5484,5485,5486,5487,5488,5489,5490,5491,5492,5493,5494,5495,5496,5497,5498,5499,5500,5501,5502))",
        "cost_info": {
          "read_cost": "32.65",
          "eval_cost": "0.96",
          "prefix_cost": "33.61",
          "data_read_per_join": "230"
        },
        "used_columns": [
          "product_id",
          "price",
          "percentage_discount",
          "lower_limit",
          "usergroup_id"
        ],
        "attached_condition": "((`gaseus`.`cscart_product_prices`.`lower_limit` = 1) and (`gaseus`.`cscart_product_prices`.`usergroup_id` in (0,1)))"
      }
    }
  }
}