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 (
    43573, 43274, 43273, 43272, 43271, 43270, 
    43269, 43268, 43267, 43266, 43265, 
    43264, 43263, 43262, 43261, 43260, 
    43259, 43258, 43257, 43256
  ) 
  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 0.00074

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_product_prices range usergroup,product_id,lower_limit,usergroup_id usergroup 9 40 Using where

Result

product_id price
43256 1.69000000
43257 1.69000000
43258 1.69000000
43259 1.69000000
43260 1.69000000
43261 1.69000000
43262 1.69000000
43263 1.69000000
43264 1.69000000
43265 1.69000000
43266 1.69000000
43267 1.69000000
43268 1.69000000
43269 1.69000000
43270 1.69000000
43271 1.69000000
43272 1.69000000
43273 1.69000000
43274 8.99000000
43573 1.69000000