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 (
    77318, 77094, 76484, 77518, 77246, 77080, 
    77507, 76443, 77045, 76891, 76890, 
    76264, 76889, 76152, 76151, 76150, 
    76149, 76148, 76147, 76146
  ) 
  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 0.00075

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
76146 1.69000000
76147 1.69000000
76148 1.69000000
76149 1.69000000
76150 1.69000000
76151 1.69000000
76152 1.69000000
76264 1.69000000
76443 1.69000000
76484 1.69000000
76889 1.69000000
76890 1.69000000
76891 1.69000000
77045 1.69000000
77080 1.69000000
77094 1.69000000
77246 1.69000000
77318 1.69000000
77507 1.69000000
77518 1.69000000