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 (136) 
  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 0.00030

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.00367808,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_prices",
          "access_type": "ref",
          "possible_keys": [
            "usergroup",
            "product_id",
            "lower_limit",
            "usergroup_id",
            "wdbi_product_id_lower_limit_usergroup_id_price_eff1727beeef",
            "wdbi_usergroup_id_lower_limit_product_id_92b7e8f6a250",
            "wdbi_product_id_lower_limit_usergroup_id_price_perc_3b2301d551d0",
            "wdbi_lower_limit_usergroup_id_product_id_b93629431718"
          ],
          "key": "wdbi_product_id_lower_limit_usergroup_id_price_perc_3b2301d551d0",
          "key_length": "6",
          "used_key_parts": ["product_id", "lower_limit"],
          "ref": ["const", "const"],
          "loops": 1,
          "rows": 2,
          "cost": 0.00193929,
          "filtered": 2.583979e-4,
          "attached_condition": "cscart_product_prices.usergroup_id in (0,1)",
          "using_index": true
        }
      },
      {
        "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
        }
      }
    ]
  }
}