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 (
    108078, 108077, 108058, 108056, 108417, 
    108060, 108850, 108963, 108962, 108386, 
    108063, 107976, 108380, 108171, 108128, 
    107947, 108896, 108895, 112381, 108982
  ) 
  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 0.00079

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
107947 1.69000000
107976 1.69000000
108056 1.69000000
108058 1.69000000
108060 1.69000000
108063 1.69000000
108077 1.69000000
108078 1.69000000
108128 1.69000000
108171 1.69000000
108380 1.69000000
108386 1.69000000
108417 1.69000000
108850 1.69000000
108895 1.69000000
108896 1.69000000
108962 1.69000000
108963 1.69000000
108982 1.69000000
112381 1.69000000