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 (
    99218, 98671, 98706, 98162, 98629, 98621, 
    98620, 98619, 98614, 98611, 98610, 
    98594, 98482, 98439, 98438, 98429, 
    98180, 97985, 97983, 97982
  ) 
  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 0.00065

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
97982 1.69000000
97983 1.69000000
97985 1.69000000
98162 1.69000000
98180 1.69000000
98429 1.69000000
98438 1.69000000
98439 1.69000000
98482 1.69000000
98594 1.69000000
98610 1.69000000
98611 1.69000000
98614 1.69000000
98619 1.69000000
98620 1.69000000
98621 1.69000000
98629 1.69000000
98671 1.69000000
98706 1.69000000
99218 1.69000000