SELECT 
  price 
FROM 
  cscart_product_prices 
WHERE 
  product_id = 109 
  AND lower_limit = 1 
  AND usergroup_id = 0 
ORDER BY 
  lower_limit ASC 
LIMIT 
  1

Query time 0.00013

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_prices",
          "access_type": "const",
          "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": "usergroup",
          "key_length": "9",
          "used_key_parts": ["product_id", "usergroup_id", "lower_limit"],
          "ref": ["const", "const", "const"],
          "rows": 1,
          "filtered": 100
        }
      }
    ]
  }
}