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 (
    101358, 101703, 101984, 101983, 101944, 
    101921, 101842, 101841, 101839, 101838, 
    101837, 101836, 101835, 101834, 101833, 
    101832, 101831, 101830, 101829, 101828
  ) 
  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 0.00061

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
101358 1.69000000
101703 1.69000000
101828 1.69000000
101829 1.69000000
101830 1.69000000
101831 1.69000000
101832 1.69000000
101833 1.69000000
101834 1.69000000
101835 1.69000000
101836 1.69000000
101837 1.69000000
101838 1.69000000
101839 1.69000000
101841 1.69000000
101842 1.69000000
101921 1.69000000
101944 1.69000000
101983 1.69000000
101984 1.69000000