I'm thinking of going to FOSDEM (last weekend in January) and my cheapest plane connection is by the Charleroi airport. It seems that I would pay just about the price of plane tickets again for the transportation to city center. Assuming that I arrive on Friday on 21:00, is there a way I could reach Midi for cheaper than 14 EUR? Riding a bicycle, renting a car or walking doesn't sound like an option to me.
Yes. The Brussels City Shuttle starts from €5 when booking early, but quickly rises to €14. This is probably what you found and why you ask for cheaper than €14.
When the Brussels City Shuttle is at €14, a cheaper alternative is the Euro Coach Shuttle. They always ask €10, even when paying on the spot. The last buses leave at 21:45, 22:45 and 23:30. A return (only bookable beforehand, not on the spot) is available for €16. Disadvantage of booking beforehand is that you commit to a certain bus. I never tried to board a different bus using a booked ticket, but it might work because the buses are always rather empty (in my experience at least).
Make sure that you remember the Euro Coach Shuttle logo (or print it), because the Brussels City Shuttle it way better signposted and that is where you will end up if you follow the natural flow.
Other options (apart from hitching a ride, but I doubt it will be easy) will be more expensive.
UPDATE: Unfortunately, this answer is outdated. The Euro Coach Shuttle stopped operating on 01/01/2016. From their website:
FROM 1st JANUARY 2016, WE STOP THE "EURO COACH SHUTTLE", thus it is not possible anymore to book tickets on our web site
The Brussels City Shuttle (operated by Flibco) will cost you 14
EUR at most. Indeed you can also find cheaper fares starting from 5
EUR by booking long in advance.