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 (
    298, 315, 236, 305, 70, 5556, 77, 79, 67, 
    83, 86, 85, 5477
  ) 
  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 0.00038

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_prices",
      "access_type": "range",
      "possible_keys": ["product_id"],
      "key": "product_id",
      "key_length": "9",
      "used_key_parts": ["product_id", "lower_limit", "usergroup_id"],
      "rows": 26,
      "filtered": 100,
      "index_condition": "cscart_product_prices.lower_limit = 1 and cscart_product_prices.product_id in (298,315,236,305,70,5556,77,79,67,83,86,85,5477) and cscart_product_prices.usergroup_id in (0,1)"
    }
  }
}

Result

product_id price
67 6.49000000
70 6.49000000
77 6.49000000
79 6.49000000
83 6.49000000
85 6.49000000
86 6.49000000
236 6.49000000
298 6.95000000
305 11.95000000
315 15.95000000
5477 8.95000000
5556 12.95000000