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 (
    101689, 101688, 102270, 102269, 102268, 
    101746, 101745, 101744, 101743, 101615, 
    101614, 101960, 101569, 101568, 101567, 
    101566, 101540, 101539, 101969, 102183
  ) 
  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 0.00060

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
101539 1.69000000
101540 1.69000000
101566 1.69000000
101567 1.69000000
101568 1.69000000
101569 1.69000000
101614 1.69000000
101615 1.69000000
101688 1.69000000
101689 1.69000000
101743 1.69000000
101744 1.69000000
101745 1.69000000
101746 1.69000000
101960 1.69000000
101969 1.69000000
102183 1.69000000
102268 1.69000000
102269 1.69000000
102270 1.69000000