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 (
    102379, 102378, 102377, 102376, 102375, 
    102374, 102373, 102372, 102371, 102370, 
    102369, 102348, 102346, 102345, 102344, 
    102343, 102342, 102341, 102340, 102339
  ) 
  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 0.00072

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
102339 1.69000000
102340 1.69000000
102341 1.69000000
102342 1.69000000
102343 1.69000000
102344 1.69000000
102345 1.69000000
102346 1.69000000
102348 1.69000000
102369 1.69000000
102370 1.69000000
102371 1.69000000
102372 1.69000000
102373 1.69000000
102374 1.69000000
102375 1.69000000
102376 1.69000000
102377 1.69000000
102378 1.69000000
102379 1.69000000