[UPDATE 29th June 2014: Do not book through Air Fast Tickets. They have been suspended by IATA for unpaid debts. You will not see your ticket, and probably your money again!]
While there's no minimum spend, nor expiry date, AF/KL tickets tend to start at €98 for a return trip to somewhere in Europe. Typically the cheapest flights will be in the Western Europe area, closest to Paris and Amsterdam. Here's what I found by selecting a couple of random dates.
Amsterdam to Paris:
Dusseldorf to Amsterdam:
A couple of drawbacks:
1) It is not always easy to tell if you are already receiving your €50 discount unless your airfare obviously cheap, like the two examples above. So it is wise to compare against the equivalent flight somewhere else. I recommend Google Flights.
2) This Online Travel Agency has not got the most fantastic reputation and has in the past cancelled some people's itineraries, though providing a refund. There are many people who get along fine with this website too, but don't be completely surprised if you run into a few difficulties.