SELECT 
  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 
  lower_limit <= 1 
  AND cscart_product_prices.product_id = 127 
  AND cscart_product_prices.usergroup_id IN (0, 0, 1) 
ORDER BY 
  lower_limit DESC 
LIMIT 
  1

Query time 0.00021

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_prices",
      "access_type": "ref",
      "possible_keys": ["usergroup", "product_id", "lower_limit", "usergroup_id"],
      "key": "product_id",
      "key_length": "3",
      "used_key_parts": ["product_id"],
      "ref": ["const"],
      "rows": 1,
      "filtered": 2.614265919,
      "attached_condition": "cscart_product_prices.lower_limit <= 1 and cscart_product_prices.usergroup_id in (0,0,1)"
    }
  }
}