SELECT 
  cscart_product_prices.product_id, 
  COALESCE(
    cscart_master_products_storefront_min_price.price, 
    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 
  LEFT JOIN cscart_master_products_storefront_min_price ON cscart_master_products_storefront_min_price.product_id = cscart_product_prices.product_id 
  AND cscart_master_products_storefront_min_price.storefront_id = 1 
WHERE 
  cscart_product_prices.product_id IN (821) 
  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 0.00029

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.00368556,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_prices",
          "access_type": "range",
          "possible_keys": [
            "usergroup",
            "product_id",
            "lower_limit",
            "usergroup_id"
          ],
          "key": "usergroup",
          "key_length": "9",
          "used_key_parts": ["product_id", "usergroup_id", "lower_limit"],
          "loops": 1,
          "rows": 2,
          "cost": 0.00194677,
          "filtered": 0.425531924,
          "attached_condition": "cscart_product_prices.product_id = 821 and cscart_product_prices.lower_limit = 1 and cscart_product_prices.usergroup_id in (0,1)"
        }
      },
      {
        "table": {
          "table_name": "cscart_master_products_storefront_min_price",
          "access_type": "const",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "8",
          "used_key_parts": ["product_id", "storefront_id"],
          "ref": ["const", "const"],
          "loops": 1,
          "rows": 1,
          "cost": 0.00173879,
          "filtered": 100
        }
      }
    ]
  }
}