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 (
    100273, 100272, 100271, 100270, 100269, 
    100268, 100267, 100266, 100265, 100264, 
    100263, 100262, 100261, 100260, 100166, 
    100165, 100164, 99754, 99738, 99737
  ) 
  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 0.00143

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
99737 1.69000000
99738 1.69000000
99754 1.69000000
100164 1.69000000
100165 1.69000000
100166 1.69000000
100260 1.69000000
100261 1.69000000
100262 1.69000000
100263 1.69000000
100264 1.69000000
100265 1.69000000
100266 1.69000000
100267 1.69000000
100268 1.69000000
100269 1.69000000
100270 1.69000000
100271 1.69000000
100272 1.69000000
100273 1.69000000