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 (
    5618, 5676, 5661, 5634, 5619, 6216, 2458, 
    2457, 2456, 2455, 2454, 2451, 2450, 
    6164, 2443, 2448, 2447, 2446, 2445, 
    2444, 85065, 85064, 78818, 78817, 2441, 
    2440, 2439, 5885, 2442, 78201, 5897, 
    2434, 2435, 2437, 2438, 2436, 6059, 
    2433, 2432, 2431, 2430, 6189, 6130, 
    6129, 6126, 6125, 6124, 6123, 6122, 
    5940, 2428, 2427, 2426, 2425, 6183, 
    2422, 2423, 2421, 2420, 2424, 2419, 
    2418, 2416, 2414, 6181, 5985, 77579, 
    6321, 26680, 6174, 6306, 2408, 2412, 
    2413, 2411, 2410, 2409, 2407, 6186, 
    103458
  ) 
  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 0.00206

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 9 160 Using where

Result

product_id price
2407 1.69000000
2408 1.69000000
2409 1.69000000
2410 1.69000000
2411 1.69000000
2412 1.69000000
2413 1.69000000
2414 1.69000000
2416 1.69000000
2418 1.69000000
2419 1.69000000
2420 1.69000000
2421 1.69000000
2422 1.69000000
2423 1.69000000
2424 1.69000000
2425 1.69000000
2426 1.69000000
2427 1.69000000
2428 1.69000000
2430 1.69000000
2431 1.69000000
2432 1.69000000
2433 1.69000000
2434 1.69000000
2435 1.69000000
2436 1.69000000
2437 1.69000000
2438 1.69000000
2439 1.69000000
2440 1.69000000
2441 1.69000000
2442 1.69000000
2443 1.69000000
2444 1.69000000
2445 1.69000000
2446 1.69000000
2447 1.69000000
2448 1.69000000
2450 1.69000000
2451 1.69000000
2454 1.69000000
2455 1.69000000
2456 1.69000000
2457 1.69000000
2458 1.69000000
5618 1.69000000
5619 1.69000000
5634 1.69000000
5661 1.69000000
5676 1.69000000
5885 1.69000000
5897 1.69000000
5940 1.69000000
5985 1.69000000
6059 1.69000000
6122 1.69000000
6123 1.69000000
6124 1.69000000
6125 1.69000000
6126 1.69000000
6129 1.69000000
6130 1.69000000
6164 1.69000000
6174 1.69000000
6181 1.69000000
6183 1.69000000
6186 1.69000000
6189 1.69000000
6216 1.69000000
6306 1.69000000
6321 1.69000000
26680 1.69000000
77579 1.69000000
78201 1.69000000
78817 1.69000000
78818 1.69000000
85064 1.69000000
85065 1.69000000
103458 1.69000000