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 (
    109734, 109733, 109732, 109731, 109730, 
    109729, 109728, 109727, 109726, 109725, 
    109724, 109723, 109722, 109721, 109720, 
    109719, 109718, 109717, 109716, 109674
  ) 
  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 0.00049

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
109674 1.69000000
109716 1.69000000
109717 1.69000000
109718 1.69000000
109719 1.69000000
109720 1.69000000
109721 1.69000000
109722 1.69000000
109723 1.69000000
109724 1.69000000
109725 1.69000000
109726 1.69000000
109727 1.69000000
109728 1.69000000
109729 1.69000000
109730 1.69000000
109731 1.69000000
109732 1.69000000
109733 1.69000000
109734 1.69000000