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 (
    122285, 122284, 122283, 122282, 109540, 
    109539, 109538, 109537, 109536, 109535, 
    109534, 109533, 109532, 109531, 109530, 
    109529, 109528, 109270, 109808, 109735
  ) 
  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
109270 1.69000000
109528 1.69000000
109529 1.69000000
109530 1.69000000
109531 1.69000000
109532 1.69000000
109533 1.69000000
109534 1.69000000
109535 1.69000000
109536 1.69000000
109537 1.69000000
109538 1.69000000
109539 1.69000000
109540 1.69000000
109735 1.69000000
109808 1.69000000
122282 1.69000000
122283 1.69000000
122284 1.69000000
122285 1.69000000