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 (
    79485, 79484, 79483, 79482, 79481, 79480, 
    79479, 79473, 79472, 79475, 79474, 
    78864, 79371, 79362, 78850, 78630, 
    79249, 79248, 79247, 79243
  ) 
  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 0.00053

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
78630 1.69000000
78850 1.69000000
78864 1.69000000
79243 1.69000000
79247 1.69000000
79248 1.69000000
79249 1.69000000
79362 1.69000000
79371 1.69000000
79472 1.69000000
79473 1.69000000
79474 1.69000000
79475 1.69000000
79479 1.69000000
79480 1.69000000
79481 1.69000000
79482 1.69000000
79483 1.69000000
79484 1.69000000
79485 1.69000000