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 (
    103976, 103975, 103974, 103973, 103972, 
    103971, 103970, 103969, 103968, 103967, 
    103966, 103965, 103964, 103963, 103962, 
    103961, 103960, 103959, 103947, 103946
  ) 
  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 0.00068

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
103946 1.69000000
103947 8.99000000
103959 1.69000000
103960 1.69000000
103961 1.69000000
103962 1.69000000
103963 1.69000000
103964 1.69000000
103965 1.69000000
103966 1.69000000
103967 1.69000000
103968 1.69000000
103969 1.69000000
103970 1.69000000
103971 1.69000000
103972 1.69000000
103973 1.69000000
103974 1.69000000
103975 1.69000000
103976 1.69000000