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 (
    110603, 110602, 110601, 111930, 111776, 
    111775, 111774, 111773, 110748, 110747, 
    110597, 110596, 110595, 110594, 110593, 
    111559, 111558, 110575, 110574, 110573
  ) 
  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 0.00079

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
110573 1.69000000
110574 1.69000000
110575 1.69000000
110593 1.69000000
110594 1.69000000
110595 1.69000000
110596 1.69000000
110597 1.69000000
110601 1.69000000
110602 1.69000000
110603 1.69000000
110747 1.69000000
110748 1.69000000
111558 1.69000000
111559 1.69000000
111773 1.69000000
111774 1.69000000
111775 1.69000000
111776 1.69000000
111930 1.69000000